THE PHONY SMILE is one of those little hypocrisies without which there would probably have been a nuclear war by now. It is essential at summit meetings and at occasions where grape drink is spilled on your sofa by a friend's 4-year-old. While its genuineness is often suspect, it can nevertheless be accepted at face value (literally), since there's no scientific way to measure the sincerity of a smile.

Or so we believed, anyway, until a few days ago, when we read in The New York Times that some California psychologists are working on ways to do just that -- measure and classify smiles, separating out the real ones from the pained facsimiles. "All smiles are not the same," says Paul Ekman of the University of California medical school in San Francisco. "A polite smile or a forced one produces a different muscle pattern than does a spontaneous smile."

To illustrate some of the findings of Dr. Ekman and his colleagues, The Times published four photographs of a woman displaying different smiles. All seem pleasant enough, but The Times explains that only one is "a genuine happy smile," distinguishable by the fact that "the muscle around the eye called the orbicularis oculi tightens to create crow's-feet while another muscle, zygomatic major, pulls the lip corners upward." In false smiles, by contrast, "the muscle between the eyebrows tends to furrow slightly." In one of them, "a trace of sadness is apparent as the triangularis muscle pulls the lip corners down." In the two other photographs, "disgust is revealed by the upper lip, which is raised by the muscle levator labii superioris."

The authors of the smile research believe it will in time prove useful to physicians and psychotherapists by helping them determine whether their patients are masking inner anguish with a smile. Meanwhile, we're keeping a close eye this week on Mr. Reagan's zygomatic major and Mr. Gorbachev's orbicularis oculi -- and hope against hope that there's no activity in the levator labii superioris of either.