How to tell if you are upper class; Class, by Jilly Cooper. New York: Alfred A. Knopf. $12.95.

Have the class barriers broken down in England? That was the question Barbara Cartland, romantic novelist and step-grandmother of Britain's future queen, was asked on the "Today" show in New York. Her answer was a magnificent illustration of the fuzziness that obscures the issue in her (and my) native land.

"Of course they have," she told Sandra Harris (we do hope it was tongue in cheek), "or I wouldn't be sitting here talking to someone like you."

Jilly Cooper has no doubts: She is convinced that class does, in fact, exist and has a book full of anecdotes to prove it. There's the very grand old woman who was told a friend was trying to attract her attention. "'That's not a friend," snorted the old lady, 'he's a doctor.'" And there's the cleaning woman whose daughter stopped speaking to her when she married into "the professional classes." What did new son-in-law do? " 'Oh,' said the daily woman, 'he's an undertaker.'"

Any Britons who managed to convince themselves that class doesn't exist must have shaken by the census form that divides the population into five "social classes" according to occupation. (Since this book was finished, the 1981 census form unkindly demoted postmen from Class III to Class IV.) Jilly Cooper, on the other hand, has divided her (and my) fellow countrymen into six categories and invented a typical family to represent each group. Of course, once you start talking about "typical" and "average" you are in trouble, since, obviously, the typical doesn't exist. (Whoever met a family with 2.7 children?)

Besides, there is no way to describe these average social types without sounding snobbish yourself or making every family sound extremely unpleasant. All the same Ms. Cooper manages to be funny, entertaining, and informative, though only too often she is also "cheeky" or "rude" in the lower-middle-class sense -- "a bit risque."

Those on the very top and the very bottom -- where nobody bothers to be pretentious -- are Ms. Cooper's favorites, though they don't escape her sharp tongue. Among those at the top is the Marquess of Londonderry, who threw soup at a fly "that was irritating him in a restaurant"; and an "imperious peer" who missed a train and "ordered the station-master to get him another one." As for those at the very bottom, "Traditionally working- class virtues are friendliness , cooperation, warmth, spontaneity, a ready sense of humor and neighbourliness. . . . That 'love' is still the most common form of address really means something."

Remember how Nancy Mitford's "Noblesse Oblige" explained the way U (upper class) and non-U (not upper class) words betray a person's class? Ms. Cooper's no-no (or non-U) words include: sports jacket, sweater, raincoat (instead of mackintosh), wallet, handbag, pardon, ("don't say pardon, say surrey"), refreshments, diapers. Very discouraging for an American.

In addition, apparently no red-blooded blue blood would dream of jogging, buying sliced bread, or rolling his sleeves above the elbow.

On the other hand there is something disarming about the aristocrat who confessed, "I can get on perfectly well with the people my children marry. What I find difficult is dogs-in-law."

I wish that Cooper's book had been carefully proofread ("Jason" switches to "Jison," a woman "dies" her hair, and an "i" has dropped out of "liaison"). I am more disappointed that nothing was done in the American edition to help the American reader (what's a "bab-gro"? And raise your hands, you Americans who understand the British educational system -- or care about it as much as Ms. cooper expects you to).

All carping aside, it would be too bad if occasional irritation or frequent laughter obscurred the important point Cooper is making: "In the 19th century, the difference between the old bonded gentry and the manufacturing classes was that the first inherited their money, the second earned it. . . . This is crucial to the understanding of English class attitudes. It underpins the idealisation of the amateur, it explains why money went on . . . beautiful things, rather than back into business.It explains the horror of trade. . . ."

Evelyn Waugh's response was: "You should not have said that aristocrats can't make money in commerce, but that when they do they become middle-class."

If many Britons agree, that sentiment might e xplain what is happening to Britain today.