My approach has changed; I've resolved myself to the reality that, for whatever reason, she finds my cuckold fantasy repulsive. But, she and I both know that she is going to cheat again. I knew this going in. She's cheated on every guy she's ever had a serious relationship. I think it satisfies some self confidence this with her. I also think that she feels better about some strange guy that she won't see every day satifying her submissive desires. This way she canhave self respect on a daily basis, then get used like a whore and feel desired whenever she wants to.
So, to answer your question, I just don't tell her I know when she's cheated. I don't confront and "catch" her. I enjoy it in, uuuuuuh, personal ways.
In the long term, this might be ahort sighted. But, like I said, she's always cheated.