Well, it may not be true in the sense that society generally says it’s true Oh, he’s dance major, he must be gay. Look at that butch hair cut, she must be gay. but I wouldn’t be surprised if there were simply different hormonal cues, which makes sense. Humans, for the most part, have learned to ignore our instincts and the cues that are sent to our brains unless we are absolutely bludgeoned. It does make sense, however, that someone with a certain sexuality sends out different cues than someone with a different sexuality. Sometimes I wonder when I see a woman that I am so wildly attracted to that maybe this is so because she might be attracted to me or, at the very least, other women. The same is true of men.
If a gay woman, a gay man, and a straight man are in a room together, the straight man might be able to tell if the gay man is gay because the gay man’s hormones will probably more heightened in the other man’s direction whereas the gay woman’s hormones will not be heightened at all. This could also be connected to the myth that bisexuals pansexuals, omnisexuals, etc. are attracted to everyone because, on a hormonal level, they respond to the hormones sent out by both or all sexes.
In short, yes, I believe gaydar is real but only a purely hormonal level.