Basically, you can't know unless they tell you. People do have 'gaydars', some better than others, but these just work by observing and putting together a whole bunch of someone's tendencies and characteristics. There are always straight people who display 'gay' tendencies (and gay people who don't), so gaydars are never 100% accurate. Anything you get from it would just be an assumption. If you want to find out if he is gay, you'll have to ask him or prompt him to tell you. Tell him you are gay/bisexual/whatever and this would give him a perfect opportunity to tell you in return.
you could try having sex with him. if he seems to enjoy it, he's probably gay. if he just asks for money, he could just be gay-for-pay
My day has been made; thank you. Yeah, there is also the idea of dropping hints about yourself that might get him to catch on if he's gay (depending on how subtle you are), and maybe if that's the case, he'll bring it up? It's really an impossible situation, sometimes.
Like Ridiculous said, the only way for you to know for sure is if he tells you. Yea, you can try to make educated guesses based on his observed behavior, but that's all just speculation.
If you come out to him first, hes more likely to feel comfortable coming out to you. And then you can move on from there.