And yes, I did make this poll to gloat, my gaydar is literally impeccable. As close to it as can be. I literally know right off the bat because if a guy is hot and straight, I lose interest immediately. I never got more than 2 wrong on the OK Cupid gay test with men and usually get zero-to-one wrong.
It's an aura. There are subtle mannerisms non-straight men have too. A lot of straight men have mannerisms that people consider stereotypically gay (hence the term stereotypical) but there's a certain sort of flair about it when it's a non-straight guy. It's not even femininity, it's not about being fem, but there's a certain presence. The way a guy carries himself with his body. And I can easily tell a masculine gay/bi man from a masculine straight man. They wear their masculinity differently.
Eye contact is a huge thing. You can tell pretty easily by where a guy traces his eyes. You can seduce a guy easily this way. The closeted types are VERY easy to catch this way, more so than even the out gay/bi guys. If he stares in your eyes or directly at you with full attentiveness for more than about 3-4 seconds, then there is a question there.
For the most part, gay/bi guys are also quicker to hug. Straight men are more reluctant to physical contact. That's something to look for, if a guy is affectionate and how affectionate is he. Gay men love to be affectionate overall, and the majority of bisexual guys I know are pretty affectionate. A lot bisexual guys however can be like straight guys in affection level, and some straight guys can be like the average gay man, so this is something you have to be really in-tune to figure out. And even then, you'll make some mistakes.
Women, it's a different story. Not too good at differentiating between straight/bi/lesbian girls
I don't know if these little clues work with girls.