It's the easy choice to say break off the engagement. What I say is that you call in sick one Friday (or take a personal day) without telling your fiance. Leave home like you're going to work, yet come back within an hour or so and quietly watch your building to see who goes in or out. If you suspect the cheater is there, get a camera, and record video evidence of who the guy is. This is in case your fiance turns out to be a real ***ch and tries to turn people against you and say you're lying or having cold feet or whatever. Because someone who is cheating typically doesn't care enough about the feelings of the other person well, not to. Feel free to show the recording to any of her relatives or friends that doubt you. Cheaters shouldn't prosper, and don't rely on anyone else to be the instrument of karma when you can do so yourself.

Well, guess it's time to break off the engagement.