How can I tell if he likes me, but he has a girlfriend?

Talk to him. Ask if he's seeing anyone.

Well, that's happened to me recently with a guy who had a girlfriend. And, I don't really know if he just really likes me but has a girlfriend at the sametime ? ... It's just kinda frustrating.
What I would ask you to do if you consider this as a good advice, is that if he really likes you, let go of him for a while. Stop the contact, and just stop talking to him, like your a little miss independant. Then if he really likes you, he just won't let you go, because...well, he likes you, and just does not wanna loose you.
That's what happende to me. I was wondering if this guy really liked me, ans I still wonder today, because he just won't let me go... BUT has a girlfriend. It's messed up , huh ? LOL XD

Anyway, that's my advice to you. Hope it helped you. =)

ask.

he will act like he does not have a girlfriend. He will flirt with you, hit on you,etc. Maybe you will catch him looking at you a lot.