Should I wait to see if he cheats?

Even if he isn't cheating, he's definitely flirting... which he shouldn't be doing since he's freaking engaged to another woman! In my opinion, he's acting really really shady and I wouldn't give him the opportunity to cheat (you can't even be sure you'll catch him when he does it). I'd address the situation right away and make it very clear that if he so much as THINKS about another woman, his *** is out the door. It's very simple, he's the one messing things up and there's not reason for you to be subtle about it. Give it to him straight and tell him that his communication with this girl is becoming very uncomfortable for you so he better do something to alter it.