Look to see how he behaves around you, how close he sits/stands next to you when he's around, how he looks at you, and such. Does he treat you differently? Does he give you a lot of attention? Does he always steal glances from you? Just try noticing stuff like that. You can even get your friends involved in this if you want because sometimes an objective person will be better at catching these signs. Also any sign of flirting can be a good sign.
But if you really want to know for sure, you can just ask him. It'll be weird and awkward, but don't think about that when you ask.