You do what I did......I called his ex and her and I went to his house, confronted him and dumped him together!! Her and I are now great friends!
how do you know if your boyfriend has cheated
That is a decision to be made between your boyfriend and yourself
...what?
No.
Why not, he is your ex. So technically he can go out with whom he wants.
tell her that her boyfriend cheated on her. she needs to know.
I think your question lacks information. For example, does your boyfriend and his ex have kids or any other reason for talking? Also, are you saying he has cheated on you with her?
I married my ex-wife after 2 yrs of divorce and i new of her infidelity
I do not have a boyfriend !! I know the longest relationship I had was 7 years! He cheated on me :(
If your 'ex' dumped you for an 'ex' that dumped her, chances are she's gonna get dumped again.
Personally, I wouldn't trust him as you can't rely on him.
There is no way to positively know if your boyfriend has cheated on you unless you catch him or he tells you. There may be some signs such as not being where he says he is or lying to you on who he was with.