CAVEAT EMPEROR, loosely translated as "beware of presidential candidates using free televion time," should be your watchword as you tune in tonight's "presidential debate minus one" at 10 p.m.

Although it is not mentioned in the history books, I am certain that before Abraham Lincoln set out to face Stephen Douglas in 1858, some forgotten Republican imagemaker took "Honest Abe" aside for a last-minute briefing.

"Abe," he must have said, "there will be some people from the papers in the audience so you've got to milk this event for all the free exposure you can. I've prepared a few inserts that you should work into the debate no matter what Douglas says. They are the sort of short, punchy comments that will look great in a newspaper lead."

Over the past 22 years, debates have come to be seen as a kind of unarmed combat designed to strip the artifice from presidential candidates and force them to bare their intermost thoughts and attitudes to the American people.

The high-minded ladies at the League of Women Voters believe that when a presidential candidate strides onto the naked stage for a debate, he is finally out of the clutches of his stable of speechwriters, pollsters and press secretaries. It is just him, his opponent and a team of dedicated reporters and columnists questing after the truth.

Fat chance. Whatever the question, Ronald Reagan and John Anderson have already heard it during the endless campaign. You name the subject and Reagan and Anderson have a nifty little sermon filed away in their internal TelePrompters for just the occasion. If the query is a tough one, the candidates will just use the old college exam trick and answer a different question they are prepared to handle. Like as not, no one will notice the difference.

That's because we have all grown soft in the four years since the last presidential debate. When one registers to vote, no one requires you to be able to distinguish a spontaneous comment from a predigested answer. Watching a debate is an acquired art.

If you want to avoid being gulled by the imagemakers during tonight's debate, you have two choices. Either turn on "Midnight Express" on ABC or spend the next 10 minutes mastering the following debate primer. History

The last time Reagan and Anderson appeared on the same platform was on March 13 in Chicago before the Illinois primary. That was the evening when Reagan turned to Anderson and asked in wonderment, "John, would you really find Teddy Kennedy preferable to me?"

The answer was yes, but Anderson didn't admit it until his short-lived concordat with Kennedy on the eve of the Democratic Convention. All in all, that Illinois debate was not Anderson's finest hour. He said unequivocally that night, "I'm not going to lead a third party effort."

Six months later, Reagan is the Republican nominee and Anderson . . . well . . . he's leading a third-party effort. If the past is any prologue, tonight these two veteran antagonists will find about as many areas of agreement as Billy Martin and George Steinbrenner. The Empty Chair

The League of Women Voters chickened out. Every reporter in town had already written his debate lead incorporating a clever joke about the empty chair. But while there will be no seat explicitly marked "Jimmy Carter," the president's brooding presence will be very much in the room. And, at this very moment, Reagan and Anderson are carefully honing their jokes about the man who wasn't there.

Whatever they come up with, it will p robably pale in comparison to Burton K. Wheeler, the inventor of the "empty chair" gambit.

Back in 1924, Wheeler, a maverick Democratic senator from Montana, was the vice presidential candidate on the third-party ticket headed by Robert LaFollette. Wheeler was working a Des Moines crowd into a frenzy over the iniquities of incumbent President Calvin Coolidge. Pointing to an empty chair on stage, Wheeler dramatically announced that he was going to ask Coolidge "to take this chair and tell me where he stands."

Of course, Coolidge was nowhere in the vicinity. But that didn't stop Wheeler from peppering the chair with a series of rhetorical questions on Prohibition and the other burning issues of the day. Each time, Wheeler paused for a few seconds as if he was waiting for an answer. Finally, Wheeler said, in mock exasperation, "There, my friends, is the usual silence that emanates from the White House." The Debate Mask

Ever since the simple act of running was transformed into the jogging industry, Americans have been aware that specialized equipment is needed in even the most unlikely circumstances. So it should come as no surprise that we recommend the purchase of a "debate mask" before tonight's epic encounter.

A debate is words and sentences an even, occasionally, complete thoughts. That's why paying attention to the flickering dots on the television screen is a mistake.

Unfortunately, Pierre Cardin is not yet marketing a full-line of designer debate masks. But there are easily obtainable substitutes. Try to find an open durgstore this afternoon and purchase a $4.95 sleepmask. Or, if you prefer, simply place your hands over your eyes for the duration of the debate. The President Is Missing

One of the reasons that it makes no difference that Jimmy Carter is not on the podium tonight is that after four long years, we have all learned how to recognize when Jimmy Carter goes into one of his self-serving political monologues. The telltale phrases abound.

When Carter says, "Anyone who seeks the high office of president must . . ." we know that what comes next is an unpaid political announcement. Similarly, if out of the blue, the president begins talking about "two courageous leaders, Anwar Sadat and Menachem Begin," it is transparently obvious that he is seizing upon a pretext to remind us of the only concrete accomplishment of his administration. We all know that.

But four years is a painfully long time to learn how to distinguish presidential hype. We have just six weeks to assess the character and abilities of Reagan and Anderson. That's why we have compiled a list of handy words and phrases that can help us identify those moments during tonight's debate when the two candidates are on autopilot. The Prerecorded Reagan

Ronald Reagan would not have made the last pilgrimage from "Bedtime for Bonzo" to the gates of the White House if he were not a master of ersatz political sincerity.

But, fortunately, there are a series of dead giveaways that can let you tell when Reagan is merely recycling outtakes from "The Speech" instead of actually responding to the reporter's questions.

Here are a few clues for listening to Reagan:

If he waxes autobiographical and refers to his services as president of the Screen Actors Guilds, ignore it.

If in answering a question on defense policy, he uses the phrase "restoring the vital margin of safety to America," you can rest assured that he has lifted the rest of his remarks from his August speeches to the VFW and the American Legion.

If he begins quoting some prominent Democrat, get up and get some potato chips.

If he is quoting John Kennedy on how "there can only be one defense policy for the United States and that is summed up in the word first . . ." you will know that you also have time to let out the cat.

More than most campaigns, Reagan's quest for the presidency is thematic. Almost everything he says or does seems designed to lure bluecollar Democrats to join his great crusade. It's the symbols Reagan uses, as well as his actual words, that are prerecorded.

Watch for images designed to play on the emotions of these working-class Democrats. Symbols like a home of your own, a steady job, savings in the bank, a decent neighborhood and the future of your children.

If the questioning in the debate allows Reagan to string together concepts like these, you can guess that Ed Meese and Stuart Spencer and the rest of Reagan's stable of advisers are rubbing their hands with glee backstage in Baltimore. The Anticipated Anderson

John Anderson is in love with words. They are his only weapon in a campaign devoid of money, party support or a high standing in the polls. There are moments when it takes an advanced degree in political metaphysics to decipher the real John Anderson in the fogbank of rhetoric. But preserve; it can be done.

Normally, you can assume that a politician is desperately ad libbing if his sentences go on forever with a structure that sounds like it was lifted from a bad translation of Cicero.

But with Anderson reality is reversed. He speaks in complex sentences normally, even when he is at home, with his feet up, watching "Masterpiece Theatre." The moment to tune Anderson out is when he starts talking in short, simple declarative sentences. Then -- and only then -- can you be sure that you are listening to the handiwork of media adviser David Garth.

On certain subjects, Anderson is to be ignored unless your idea of fun is reading the "Federal Register." A "snooze alert" is sounded when Anderson begins talking about his 50-cents-a-gallon gasoline tax, his National Unity campaign, the ERA or his 317-page political platform. This latter document has only been read by four political science professors and three reporters who drew combat pay for the assignment.

Growing up in Rockford, Ill., Anderson undoubtedly had a sixth-grade teacher who instilled in young John the virtue of memorizing great quotations. At the slightest provocation, Anderson will begin quoting from the likes of Ralph Waldo Emerson and James Fenimore Cooper. If Anderson begins sounding like the candidate from Bartlett's, assume that he is merely displaying his erudition for every American voter who owns a Cuisinart.

Here are some other things to ignore in Anderson's rhetoric:

If he claims that the only way that Reagan can cut taxes, increase defense spending and balance the budget is "with mirrors" remember that this is the line that made him famous during the Iowa debate in January.

If he says that "the Carter administration has allowed an oily sword of Damocles to hang over our heads," switch channels immediately to check out Turkish prison conditions on ABC.

Even Anderson's invective is likely to be recycled from the campaign trail. Recently he's been wowing campus audiences with the line, "Mr. Reagan isn't even a man for the 1950s. He's really a man of the 1920s." The Unasked Questions

By the best reckoning, there will be about six minutes of spontaneous give-and-take during the hour-long debate. The rest of the proceedings will be about as unrehearsed as Charles van Doren's appearances on that old rigged quiz show, "Twenty-One."

But it doesn't have to be like this.

The fault, I am sad to admit, lies with the reporters asking the questions.

Undoubtedly, they will be caught up in the earnest quality of the proceedings. They will ask complex questions about the future of the Western Alliance and the moribund state of the American economy. In return, they will get predictable answers with all the intellectual content of a commercial decrying "ring around the collar."

It's all a sham. If you want to see two presidential candidates stripped to the bare essentials, hope and pray that there is a reporter on the panel with the irreverence to dish up questions like these:

Governor Reagan. Portugal is a vital NATO ally controlling the crucial airbases in the Azores. In 1974 and 1975, Portugal flirted with Communism. aYou would agree I'm sure, Governor Reagan, that Portugal is of vital concern to the next president. Could you please take a few minutes to discuss the internal political situation in Portugal today? In your answer, please give the name of the current American ambassador, the current prime minister of Portugal and the names and political philosophies of at least three of the leaders of opposition parties.

Congressman Anderson. Without looking, could you please tell us what color socks you are wearing tonight? Also, who chose the socks from your wardrobe? Was it you, you wife, Keke, or David Garth? Who Cares, Anyway?

All of this might be of transcendent importance if presidential debates actually provided many useful clues as to how the winning candidate would behave in the Oval Office. Unfortunately, horoscopes have about as good a track record as the much-vaunted presidential debates.

Remember the second debate of 1976? The one where Jerry Ford said that the Polish people do not "consider themselves dominated by the Soviet Union." Pretty dumb, huh? Jimmy Carter jumped all over it, saying that even schoolchildren knew that Poland was a commie country.

Now, just four years later, Poland has free trade unions. And Russia is too mired down in Afghanistan to do much about it. That was a pretty prescient comment by Jerry Ford, now wasn't it?

So as you sit before the television screen tonight with your debate mask on, take heart. Nothing you hear tonight has any bearing on how either Reagan or Anderson might behave as president. Given the expected level of tonight's debate, that's something to feel pretty fortunate about.