Well pick one or more of these:
Tell him you're gay. He might then or later feel safe to share if he's gay or bi.
Ask him if he has a girlfriend.
Offer to go out with him drinking to meet girls.
See if he checks out guys often.
Go to a gym or swimming with him and observe.
Go to a sauna with him and observe.
Best is still:
Be brave, tell him you like him, you're gay and if there's any chance there could be more between you. Now this is very delicate but also definitive and if he's been nice to you and he's sane he'll say yes or no, sorry, dude...