This is a very subjective matter.
Even if the guy is, in fact, gay he might not be interested in you.
Or he may be willing to jump your bones but he doesn't have the courage to disclose he is gay or take initiative (a.k.a. closet cases).
I would LOVE to date a closet case but it's so difficult to smoke them out of the closet. And of course, we have to respect people's choices.
Easy way out: hit the gay bars and clubs and stick to the guys you know for sure they're gay.
If you're curious about a guy, try and date him first. Set the mood and ask if he's gay. If he is gay but is in denial, well, there's nothing you can do. He'll come out eventually under his own terms.
However, I feel that some closet cases are on the edge and they just need that extra push.. Maybe if you try and get close to his face, looking directly in his eyes. If he holds eye contact, try and kiss him. I hope you score.