President Carter has expressed the wish to have ordinary people to dinner at the White House. But it isn't going to be easy.

The same problem is bound to arise that keeps him from fulfilling his desire to have women and blacks well represented in top goverment jobs - where on earth does one find them?

It has been suggested that candidates for the position of ordinary citizen write in to the White House, stating their qualifications and previous dinner experience, if any. An essay might be required, in which the applicant could state his reasons for wanting to eat dinner, and the contribution he felt he could make to the evening, such as telling funny stories or bringing the wine.

Another idea being discussed is to choose ordinary citizens at random from the telephones book. This eliminates people who have had their names removed from the telephone book for such reasons as having been annoyed by people calling up and telling them they've just a won a free evening at the White House, with extra rhumba lessons optional.

But however the search is conducted, there is going to be trouble over the definition of ordinary. In a country where rich and poor alike identify themselves as middle class, how do you tell when someone is just plain ordinary?

By clothes, perhaps. Since blue jeans are the uniform of the upper classes, and the really chic women these days are running around in their grandmother's underwear, an ordinary person could be identified by the dress or the suit she or he has on.

Education is another giveaway. You can always tell an aristocrat form the fact that his schooling has taught him to express his feelings, whereas some ordinary people have been taught, in a modest way, to read and write. By giving a spelling test, or checking applicants' handwriting for legibility, you might eliminate the rich kids.You might also end up with an empty table, however.

Manners are different, too. Many ordinary people are aware of the concept of Best Manners, or Good Behavior, while the aristocracy recognizes only one style, Informal.

But this being a democratic country, it's extremely difficult to be sure about anyone. An ordinary person with pretensions can, with careful attention to detail, work his way up to sloppiness or rudeness or any other perogative of his betters. Perhaps the only sure way to tell an ordinary person is that he is not invited to White House dinners.

And a good thing it was, too. Suppose we were to represent ourselves to foreigners as a nation of conventionally dressed people on their best behavior?