First off I don't think a dream can definitely tell you that someone is cheating, they more than likely just reflect your own fears and doubts - which could of course be based on some real evidence that you may have noticed.

It's a difficult one though, because you have cheated before it's going to be very difficult not to think of it. Very difficult for both of you actually. It's sort of the human brain works, we often assume that other people think the same way as we do. So if we lie a lot, we assume that other people lie a lot too... or people who are habitually honest and naively nice often assume the best in others. It's hard to get a away from.

Do you think you would cheat again? I know that's not what you're asking, you want to know if your gf is cheating, but it might be all tied into why you're thinking that way. Has she cheated on anyone before, do you know?