The title of this article in New York magazine is misleading -- it's not really about how people can tell whether someone is gay or not, it's about the science surrounding all the little things that seem to go along with being gay: the tendency to have a counterclockwise hair whorl, or more tightly packed fingerprints or the greater likelihood of being left-handed or ambidextrous.
But I'm not doing the article justice -- it's filled with all sorts of fascinating research and insights. Two of my favorites:
"Because many of these newly identified “gay” traits and characteristics are known to be influenced in utero, researchers think they may be narrowing in on when gayness is set—and identifying its possible triggers. They believe that homosexuality may be the result of some interaction between a pregnant mother and her fetus. Several hypothetical mechanisms have been identified, most pointing to an alteration in the flow of male hormones in the formation of boys and female hormones in the gestation of girls. What causes this? Nobody has any direct evidence one way or another, but a list of suspects includes germs, genes, maternal stress, and even allergy—maybe the mother mounts some immunological response to the fetal hormones."
"In a universe in which we look for purpose in order to appoint value, what is the purpose of my gayness? Dean Hamer sees one possible answer in the fraternal-birth-order studies. “In Polynesian cultures, where you’re talking about very big families, it was typical to have the last-born son be mahu, or gay,” he says. Explorers described young boys who looked after the family and sometimes dressed as girls. “They suspected that their families had made them that way. But you just can’t take a guy and make him clean up and have him become gay. He’s got to have some gayness inside. Maybe that’s the biological purpose to the mahu: taking care of Mom.”