Well. Besides the obvious behaviors (like actually partaking in homosexual acts).
Guys who make excessive eye contact with me are usually dubbed as gay in my mind.
There are the obvious other things: Walk, liking Lady Gaga (among others), and other things that give suspicions; however, what's more telling is taking a closer look as to their mannerisms.
For example, word choice is a good indicator. When a guy says specifically to you that like, "Half the time it's like you're not speaking English anyway. You speak so eloquently," you can say at the very least his sexual orientation is questionable.
Other than that, I want to mention again that body language is important too. Sometimes y'know guys can be touchy and stuff and it's totally no homo, but it CAN support the case with other things (the aforementioned things).
The proximity that the guy allows you to be to him or the vice versa can give a clue too.
Inquisitive nature about you (and specifically your sexual orientation, can provide a clear case of curiosity even if not romantic and such).
Those are kinda general, but.