Usually, if you suspect a woman of cheating on you, it's because of how she treats you. So, what you should ask yourself is whether her behavior is acceptable, as long as she isn't cheating. If the answer is NO, then delving into whether she is or isn't cheating is a huge waste of time.
If it's a pattern of disrespect, over multiple relationships, the problem is probably the guy you see when you shave. Millions of parents have discipline problems with their children, but if the kids act like angels at home and terrorists at school, or vice versa, the problem is probably not a defective kid. Likewise, if your woman has tranformed from a goddess to a demon, you might need to consult the operator's manual, before you pick up a replacement model.