Thank you all for the help:)
Backstory:
My fiance and I first started dating about 5 years ago. We had a great relationship for about a year and half and then our relationship went to hell. She started manifesting some major issues like physical abuse towards me and always having to get her way, etc... Then I started getting advice on how to handle this from some chick who was going through the same thing with her boyfriend. My girl got really pissed off and thought I was cheating on her. It caused A LOT of drama.
So for about the next 6 months we tried to work through these issues. Things were pretty rough at times, but I thought things would be able to work out. Well one weekend I was out of town and after I got back, she seemed really weird. She all of a sudden was very cold toward me and seemed to have no desire to want to be with me. Then I started seeing all of these new pictures of her on myspace with some new guy. I asked her about them and she kind of blew it off and wouldn't say much. A couple of weeks of her always ditching me to "hang out with her friend", I told her that I think she was cheating on me. She denied it, but would still refuse to let me hang out with her and her friend. So finally, I told her that I want to break up. She was a little upset, but was otherwise fine with it. For a few weeks after that she would still try to call me and e-mail me. Then one morning I got a call from her and said "I told you, if you talk to him, we are done". She refused to stop so I told her "don't call me anymore". (This coincidentally inspired me to learn the pick-up arts after our relationship ended)
That was the last time we talked until 2 years later. At this point a year ago, she had gone through several more boyfriends, but still was in touch with this douce-bag. Then one night the craziest thing happened. Her and her roomate were having a house-warming party at their new place. Sure enough she invites the mother-fucker. During the party it was totally awkward. She was talkative to him, but it seemed like she paid more attention to me. Since we weren't dating at the time, I couldn't tell her that he couldn't come, but it still really pissed me off.
"I cannot understand why you're feeling uncomfortable speaking with her about this."
This particular guy is a bit of a taboo subject. She doesn't even know that I know she cheated on me with him. At the party she tried to play him off as an old friend.
This is the first time he has spoken to her since a year ago. I don't know how to bring it up to her, since she doesn't think I know as much as I do. I know I shouldn't look in her phone, but she told me she has hidden things from a different one of her ex's. I just want to make sure she is not pulling shit with me. And besides, she always checks my phone to try and catch me in a lie.
"she have'nt learn her lessons of her previous mistakes she would do it again hoping u are always there take her back with an open arms after her adventures."
Right on, this is exactly what I want to avoid
"clearly that what she is doing behind your back is not what u consider responsible and acceptable."
Is it okay to strait out tell her I have a feeling she is talking to her ex behind my back?
"If she starts acting distant, or goes out doing things she normally didn't do. If you suspect her cheating and she is, it WILL come out of the woodwork."
I will be sure to watch for that

If I notice any of these behaviors, what is the best way to confront them? Any acqusations will be made mostly on assumptions, and wouldn't that push her even further away?
thanks again!