Answer:
you can never know for sure (seeing how he's long distance, you can't possibly find out unless he was stupid enough to tell you). but if you're talking about signs like he's distant (mentally speaking) and doesn't seem so excited to be talking to you, that may not be cheating, but it's not good. so if he's doing anything like that, whether he's cheating on you or not, it should just be ended because something is obviously wrong.