How can you tell your boyfriend's girlfriend that he is cheating on her with you?

Answer:

I think you have to first ask yourself what your motives are. If you are trying to keep this guy. Then don't tell. Although, honestly, why you would want a man like that would be the subject of another answer.
If you have decided that you deserve better and do not want him in your life anymore--then go for it and tell her. Would you want to be her? Wouldnt you hope that someone would have the balls to tell you if your man was cheating on you with someone else? Also, I think telling her calls him out on the terrible things that he has done both to you and to her and god knows what other women he's hurt. Maybe it will show him that his selfish actions have consequences. Also, in a way, it allows you to get some of your own dignity back--especially if you didn't know about the girl in the first place. You'll send a clear message that you are not going to sit around while some man runs carte blanche through everyones life indulging his selfish whims at the emotional expense of others.
Be prepared though. She will not believe you. You should be able to have some sort of proof exp. emails etc. Even then she still may not believe you. You should also be prepared for the man to never talk to you again. But if your motive was true, and you weren't doing it to break up the relationship so you could have him...or some other sort of revenge...then telling is the right thing to do in my opinion. If everyone had the attitude "it's not my business" when they witness horrible things--what kind of world would we live in?  
If they smell different you know they are cheating on you
also if they start scratching in that place alot *omg they had sex with someone esle :-O*