The term "gaydar" has been around for a while now. It is when one person (usually gay) looks at another person and gets the sense that other person is gay. This happens without the second person saying "I'm gay." I've had that experience over the years, though I can't say how accurately my gaydar is calibrated because I don't go up to others and ask, "Are you gay?"
William Cox led a study on gaydar at the University of Wisconsin in Madison. He says gaydar is only a reliance on problematic stereotypes. And stereotyping is not appropriate, even if you rename it gaydar in hopes of making the idea more acceptable.
Charles Pulliam-Moore thinks Cox misses the point. Gaydar may not work much better than randomly guessing. But there is a deeper usefulness. Believing gaydar exists allows them to see their space as queer-affirming. It allows a closeted queer to feel less alone. It helps us to construct our own queer identity.