One way to tell if he likes you is by payiing close attention to how he acts around you. However, if you really want to ask him, you may want to keep it casual and act as though you're asking him as a friend. I personally recomend that you don't ask him up front unless you want to make a big deal out of the situation, but your aproach, whichever one you use, totally depends on your relationship with the guy. If you don't really know him, you might want to get to know him at least as an aquaintance and then ask him. If you are buddy-buddy with him, you could ask him up front or ask him casually. It all depends on what kind of person you are and how you want everything to play-out.