Things had been on the slide for some time. And on that damp and dreary March morning, the smell of gasoline and oil at the Capitol Hill Exxon station told the red-haired computer specialist that her romantic days with the Hill aide were numbered.

The willowy woman, her face flushed from the cold and the fight they had just had, stared glumly across the Toyota at her rugged-looking boyfriend. She watched and waited. Her lower lip started to tremble as the gas station attendant approached. And then it happened. Cracking the window, the legislative aide asked the attendant to fill up the tank.

That was the end.

"Not once," she says today, "not once in the nine months we dated did he ever get out of the car at the gas station. It really told me something about him.

"I mean if a man can't even get out of a car at a gas station how does he expect to take an active part of life . . . it shows he can't interact on different social levels . . . I was right, too. Once we went to the Kennedy Center, and he was afraid to leave his seat all night . . . And when my 12-year-old brother was in town, he avoided me for a week . . . Getting out of the car at a gas station is the real test for me . . . I could never fall in love with someone who didn't . . ."

It was, in effect, The Litmus Test. And the defenseless legislative aide failed.

There are those magically enlightening moments in every affaire de coeur when the entire relationship is summed up. A seemingly insignificant gesture, an offhand comment, or a powder-blue and pink plaid jacket becomes a symbolic test that either sets the heart aflutter again or leads to an abrupt ending.

It is that flash of truth when he goes to slip the ring on her finger and realizes he can't stand the shape of her hands; when, after five months of a glorious winter wonderland affair, she sees him strut onto the beach wearing a crimson stretch bikini bathing suit; when he meets the person he's sure he wants to spend the rest of his life with -- until . . .

"I met this really intriguing woman at a party in Georgetown the other night," said a Washington musician who looks as if he should be herding cattle rather than making the Georgetown rounds. "We had so much to say to each other all night, and I really wanted to go out with her until we got to her car. It was a red Pontiac Grand Prix . . ."

"What could I possibly have in common long-term with a woman who drives a Pontiac Grand Prix?"

However weird, judgmental or idiosyncratic it appears, like laughing or crying, The Litmus Test is an emotional reality that only our romantic brain cells understand.

"The more rational litmus tests don't seem to be associated with long-term happy relationships and marriages anymore," says Dr. Ellen Frank, associate professor of psychiatry at the University of Pittsburgh, who has conducted studies of marital pitfalls. "We have found that people tend to pay more attention to these seemingly frivolous, irrational tests, and it probably leads to happier relationships in the end."

I'm suspicious of women who have more than one dog and two cats, women who read "The Prophet," women with Modigliani posters hanging on their walls (it's the biggest cliche' of all time), women who play Doobie Brothers records when they take you home, women who talk about their diets when you're trying to eat, women who at dinner parties talk about the insensitivity of their husbands, women who call you sweetheart on the second date, women who have pictures of their fathers on the dresser, women who discuss "General Hospital," and women who keep diaries -- it makes me nervous, even for a writer. -- Pete Hamill, writer

Success Story

Alejandro Orfila, secretary general of the Organization of American States, remembers Jan. 25, 1976. It was snowing and he escorted Jacqueline Onassis to the Kennedy Center's fifth anniversary celebration. "I spotted this striking blond woman from across the room, and I couldn't take my eyes off her," recalls Orfila. "It was her presence. A woman has to have an overall presence for me. All the pieces have to fit. It was her look, her walk, her smile -- they were all right."

Orfila told Onassis she would have to fend for herself for a few minutes. He grappled his way through the crowds in search of the blond, only to find she had slipped out the door. So he asked and asked and finally found someone who knew the New York model. He followed her north and wooed her. They were married one year later. "I had no need to get married at that time," he says, "but Helga just commanded that presence."

Could anything have changed his mind?

"Oh, yes, if she had a loud horrible laugh," said Orfila, demonstrating a loud horrible laugh. "I'd been out with women and all they have to do is laugh, and there was no second date."

I would summarily dismiss a man with thin lips. Where I come from we call it chicken lips. I think it indicates a stinginess of spirit. The eyes can lie. The lips can't. -- Barbara Howar

I always watch the way a man walks into a room . . . It shows whether he is confident or a put-on. I believe you can size up a man if he walks into a room like a great man . . . And I think the man should walk into the room first. Today, women want to walk in first. -- Pamela Harriman

Political Bedfellows

In Washington, where your political party is your middle name, The Political Litmus Test is critical.

"Democratic men are more fun," says a Republican senator's legislative aide. "But after a few dates we start arguing politics. So then I'll go out with a Republican man, and we'll agree politically, but he'll be boring. I just can't get a good balance."

Sometimes the differences are more subtle. Consider the case of the Carter staff aide who was seriously dating the Kennedy staff aide.

"After Kennedy announced, we had some real problems," said the former Carter staffer. "We would constantly fight, and finally one night over dinner, I told her we had better wrap it up because it was going to be a long campaign.

"Basically, at some point you come to the realization that you're in love with someone who is trying to take your job away. It just can't work because it's your whole life, what you're all about. I could never get serious about a woman who didn't agree with me politically."

I absolutely despise women who drink beer out of a paper cup -- especially since there is a semi-feminine way to drink beer out of a glass bottle. Once some friends of mine tried to fix me up with a fairly attractive woman. I met her at a picnic and as soon as I saw her drink beer out of a paper cup, I knew it couldn't work. I thought she was a sludgeball. -- a Washington writer

Once Upon a Time

It used to be a lot easier. He didn't particularly care whether or not her nail polish was nicked, and she wasn't as concerned with his style of underwear as she was with his religion, nationality and ethnic background.

But every generation has its tests. For Ginger Rogers in the 1942 film "Tom, Dick and Harry," it was bells. Playing a dizzy switchboard operator, she became engaged to three men at one time and ultimately married the one whose kisses set off bells between her ears.

A Boston housewife recalled that during her first date with her husband-to-be, he brought a box of Jujy Fruits to the movies with him. During the entire show he held up the multiflavored candies to the light and only ate the colors he liked. "I couldn't imagine marrying anyone that was so weird," she says today.

He wasn't too enamored with her at first, either. During a romantic stroll one night, he accidently fell off the curb and she laughed hysterically. "That almost did it for me," he says. "I couldn't believe she didn't have the common courtesy to keep it inside. I was embarrassed enough as it was."

Nonetheless, it wasn't enough to keep them apart. Thirty years and seven kids later, they are still together.

Maybe it was the sexual revolution that allowed the luxury of time and options to find The Right Person. The threat of becoming an old maid was left trailing behind the women's movement, and men simply no longer believe it's a sign of virility to marry quickly and start a family. Somewhere along the way dating and pairing seems to have given way to nitpicking and choosing.

Short socks. I always go right for a man's feet and if he crosses his legs and his ankles show I'd never go out with him again. There's nothing sexy about short socks, and besides, it shows he takes no pride in his appearance. -- female vice-president of a Fortune 500 company

For Openers

"For me, whether a woman leans over and opens the car lock to let you in says it all," says the young musician, echoing the sentiments of four other men interviewed. "It's a sign of consideration. If I have to open the car door myself after letting her in, it would cause a lot of problems later."

He learned this lesson the hard way.

"We're not married anymore. It came up a lot during the divorce. We're friends now. She still doesn't open the door for me."

One thing that sends me running is if a woman starts kissing her cat the minute we walk into her apartment after a date. First of all, I think that's a lot of wasted affection that could be used on me. And more important, I think anyone who has to give that much attention to a cat has a deep-rooted problem in dealing with man-woman relationships. It happened to me just the other week and I couldn't bring myself to ask her out again. She was pretty cute, too. -- Tommy Curtis, radio personality

Teeny Weeny Bikinis

Some litmus tests are more universal than others.

On the French and Italian Riviera, multicolored bikini bathing suits for men are considered sexy. But on the white, hot sands of Rehoboth or Hyannis Port, they connote sexual blatancy, tastelessness, womanizing and promiscuity.

"They are the most disgusting things I have ever seen in my life," says a Washington receptionist, who was raised in a Massachusetts beach town where if a man didn't wear faded cutoff jeans for swimming, he was ostracized all summer.

"Every June when the 'No Swimming, No Drinking, No Eating,' sign went up, we would add 'No Bikini Bathing Suits,' " said the receptionist.

But even The Litmus Tests that seem to be set in concrete from shore to shore are meant to be ignored occasionally.

Several winters back, the receptionist and a stewardess friend from Boston set out for a Martinique vacation, stopping over in Miami for what was supposed to be one day. They were only on the beach five minutes when two well-built, tanned, bikini-clad male bodies approached.

"Oh, my God," said the stewardess, "they're heading for us. Don't look at them, maybe they'll go away."

But they looked. And they liked what they saw. So the two women stayed in Miami for two weeks with the two men, who happened to be well-known baseball players for a major league team.

"We fell absolutely in love with them," says the receptionist, who still dates one of them. "We forgot about the bathing suits."

Has she ever brought the young man to her home at the beach?

"Never!" she gasps. "I know I could never marry him for that reason."

Bald men have always been a test for me. They just do something to me. They just look so vulnerable to me that I want to hug and kiss and protect them. -- an attorney married to a bald economist

The Godmothers Ltd.

Abby Hirsch is an expert at interpreting The Litmus Tests. Three years ago she started The Godmothers Ltd., a upper-crust dating service serving 1,200 love-hungry clients in New York, Philadelphia and Washington.

Hirsch says one Litmus Test that has been repeated "all too often for me to ignore" is what she calls the Nail Polish Theory. "We either get a man who says 'find me someone who wouldn't dream of having a chip in her nail polish,' which tells us they're looking for someone perfectly groomed, someone who fits into any social landscape," says Hirsch, "or a man who wouldn't dare date anyone who would wear nail polish. It means something unnatural for them . . ."

Basically, by looking at a woman's hands and her fingernails, I get a feeling for her personality. There's no rational reason. I just get an idea of how much in control of herself she is, how neat she is and how sexy she is. I prefer long nails and thin fingers. Winter is a frustrating season for me because they all wear gloves. -- a Brazilian journalist

You Are What You Wear

A woman in her early thirties recently sat next to a "gorgeous" businessman at a downtown dinner party and subsequently was reminded of one of her longstanding "horrifying tests."

"The night of the party he was dressed great, with a tailored three-piece suit and silk shirt, and he was quite interesting," she recalls. When he subsequently called to suggest a quick dinner at a trendy Washington restaurant, she was thrilled.

"I couldn't believe my eyes when I got there," she says with a shudder. "He was leaning against the bar wearing a black shirt. It was awful. I could never date anyone who wore black shirts . . . It's so disgusting . . . It's sleazy . . . It's like nails scratching on a blackboard."

She walked out of the restaurant and canceled the date from a pay phone down the street.

If I take out a cigar in front of a woman and she says "Are you going to light that thing up?" the friendship is over. There's no sense in pursuing the conversation any further. -- Art Buchwald

Scientific Proof

In 1976, the University of Pittsburgh marriage counseling center conducted a study of 131 couples to determine, among other things, what initially attracted the spouses-to-be to each other.

The results showed that those who had checked off such things as "the relationship was never dull; physically attractive, beautiful or handsome; sexually exciting; romantic; and life of the party," were still happily married.

Couples who married on the basis of such things as "intellectually challenging; dependable; maturity; financial security; and common interests," were seeking marriage counseling.

"Not very many things surprised us about that survey, but that those results was surely one of them," said Dr. Ellen Frank. "The people who selected the more romantic, as opposed to cerebral, reasons for marrying were happier by far. Personally, those results influenced the way I counsel now. These tests really can't be ignored."

So agrees the willowy, red-haired computer specialist. Shortly after her devastating breakup with the Hill aide, she met someone new. They might marry, she says.

"He always gets out of the car at the station," smiles the love-struck woman. "But I am a little concerned. I realized the other day that the gas cap on his tank is locked, so he has to get out. But he really did want to meet my brother, so I'm encouraged."