Gaydar--the supposed ability to identify a person's sexual preference just by looking at them--may be more than just a bad pun. A couple of studies have been done on this recently, as part of broader research into the general ability of humans to very quickly make correct judgments about other humans' skills, personalities, age, etc.Turns out, people asked to guess whether a man is gay or a woman is a lesbian, just by looking at the subject's face, got it right at a rate higher than that which could be explained by mere chance. And the trick worked even when the images were cropped to remove all trace of tip-offs like hair style or fashion accessories.
So far, the studies have been very small and they certainly don't show that gaydar, if it does exist, is foolproof. The guessers did statistically significantly better than chance, but they weren't exactly sexual-orientation sniffing bloodhounds. There's also some open questions about whether the gender or sexual orientation of the guesser makes a difference on the rate of accuracy. What the research does do is add to considerable body of evidence showing that humans are evolutionarily programmed to pay extremely close attention to the facial features of other humans, and it tells us that we still have a lot to learn about what that programming means for ideas like "instinct".