Most Helpful Girl
Don't confront him until you have evidence. If he's lying, the key to breaking him is to ask a lot of questions. Not in an interrogation sort of way, but just casual talking. Ask him where he was last night, get the details, and see if his story has any holes.
But like the others said, it's not a good sign when you don't trust the guy you're dating. My advice is to proceed with caution.