When Gerald Dunham took the examination to enter the U.S. Customs Service, he was asked whether it were true or false that 99 percent of the American people who pass through Customs are loyal, trustworthy and honest.

He put "false."

"I came from a Customs family," he said. "I believed that everybody was smuggling something. "

The correct answer was "true," and Dunham, now an inspector-instructor, at the U.S. Customs Academy in Georgetown, has come to believe it.

You and I may look at our fellow passengers, busily filling out their declarations, and wonder why the same people we saw bargain hunting abroad are all swearing that they didn't spend over the permitted $100. Loyal, trustworthy honest citizens who make an exception for expense account, tax returns or customs declarations.

But Carl Werner, Customs director at Dulles Airport, estimates the number of liars per planeload as "one, if any." Dulles is now putting in a system enabling 60 per cent of the passengers to pass through Customs in seconds, showing only their hand baggage. Another 15 per cent will undergo more thorough examination on imports which they have honestly declared to be in excess of the $100 duty-free limit.

This leaves 5 per cent - to include everyone from hard-core dope smugglers to first-time offenders with the idea that if they wear their new Swiss watches nobody will notice them, to perfectly honest people who somehow excited an inspector's suspicion.

One of the differences between this observations about honesty and yours is that he knows the rules. The person who declares $99 worth of merchandise when he actually spend $120 is probably within the legal limits because the value of his property is computed, for duty purposes, at the wholesale price. Also there are many items and many countries' products that are not subject to duty at all.

"We know we can't look at everything that comes through," said Dunham. "So we have to learn to look at the people."

Or, as Sidney Reyes, chief inspector at Dulles, put it, "I can't look at all 15 of your bags. So I'm going to make you tell me which one is it?"

How do you train people to do this?

"Tourist often say to me, 'Don't I have an honest face?'" Dunham said. "Now you tell me what an honest face looks like."

Whatever assumptions may be made about the prevalence of honesty, this means that everybody is suspect. The Apollo II astronauts had to be inspected when they returned from the moon, and to declare their cargo acquired abroad, which was moon rock.

"When someone has paid $1,600 to fly on the Concorde, it's because time is money to him," said Reyes, "and when he has a private plane and pilot waiting to take him on, but finds he had to stand in line behind someone with three salamis, it's going to kill him. But we treat everyone alike."

You may get a thorough examination because you visited a country known as a narcotics producer or simply as bargain paradise. Or there may just be something about you that announces you should be looked at carefully.

The inspector is trying to see if everything about the person seems to go together - the behavior, the statement, the appearance the luggage, the answers to seemingly casual questions.

Reyes offered the following examples from actual incidents at Dulles.

A woman says she's a dental technician. "Do you have to go to school for that?" asks the inspector. "Sure," comes the quick reply. "Where'd you go?" Pause, Long pause. Yes, she was smuggling.

Inspector leans toward woman passenger and says, "Look, I can smell a mango somewhere - I know you've got one." The woman replies, "Oh, no!" but claps her hand over her left breast as she says so.

A man gives an inspector a hard time because she found "just a lousy pair of $20 slacks" he didn't declare, "and she's giving me a hard time." The supervisor decides to give him a hard time, and discovers $400 worth of undeclared merchandise. And then, just to be nasty, picks the cigar out of the passenger's pocket to see if it is Cuban. It's full of hashish. The fuss over the clothing had been what is known as a diversion.

A jewler returns from a visit to his country to origin, loaded down with bulging luggage held together with rope. "Nothing to declare!" he declares. A suspicious remark, which eventually leads to the most drastic search, in which the traveler is stripped. Hundreds of gold rings are sewn into his underwear. "Son of a b . . .!" shouts the passenger. "Who put those there?"

The human clue would he any tiny detail, said Dunham, who fells the following story on himself.

"A priest came through, and I noticed he was wearing brown shoes. I thought to myself, 'He should be wearing black shoes.' I'm a Baptist - what did I know about what a Roman Catholic priest should wear? As a matter of fact, loads of them wear brown shoes.

"But I didn't know that then. So I searched him. He had 40 pounds of narcotics on him."

One thing that doesn't work is stereotypes based on age, origin, clothes or just about anything else, they all agree. The little old gray-haired lady is just as apt to be a smuggler as the punk kid is to be honest, and the more people you see, the less you believe in such things, they agree.

What does show through is behavior. "Most people are terrible liars," said Reyes. "Not only have they not prepared their stories far enough, but they show that something inside is eating them alive. I can even smell when someone is lying."

Of course the inspectors do have advantages other than personlity sniffing.An information network feeds them data about people and prices. Your falsity could be reported by anyone from a dog who can find where the dope is, to a passenger who heard you bragging on the airplane.

And the inspectors tend to know better than you did what you paid for what.

In the basic customs course, inspectors - who have to enforce something like 400 regulations made by 40 different government agencies - are taught to classify items, so they can at least look up what is what and comes under what regulation.

It can certainly be varied and confusing. Nathanial Hawthorne, who was once a Customs inspector - as were writers Herman Melville and Edwin Arlington Robinson - once wrote about his job to a friend, "I have no reason to doubt my capacity to fulfill the duties, for I don't know what they are."

The chief way of learning is on the job. Inspectors, who alternate between seeing luggage and commercial shipments, find their eyes are trained quickly. You may think you found an unusal treasure abroad, but the chances are the inspector has already seen dozens of duplicates that day.

For the training of advanced experts, there are courses and seminars in specialized fields.

Holly Kuga, whose areas include wood and paper products, textiles, chemicals, handbags and footwear, can tell you in a minute if your Oritental rug is really what you thought it was when you bought it, and when she goes to buy shoes for herself, she knows exactly how long they're going to last.

As an occupational hazard, she has picked up a taste for Oriental rugs herself, and her eye for expensive clothing gets better every day. "But there are restraints built into this," she said. "When I've inspected a shipment and I know what the store has paid, I'm not going to be able to bring myself to pay that price I see the store asking for it later."

"I can recognize a genuine antique," said Reyes, "but I can continue to call it junk forever, and say that I'm never going to put that thing in my living room."