I am/was sort of like your wife. I know you are kind of leaning towards not telling her that you know of her plans, but...think about this:
Get into the conversation with her about the w/e, telling her you hope she has fun. Tell her that you know she is just going to see her friends...however...in your gut you "cannot help, but to feel, that she is going to visit with the OM". It's just how you feel...look her in the eye. She'll protest, let her, but just say...Okay.........
That way, she doesn't think, that you actually know for sure. Trust me, it will really put her w/e in a guilty spot.
One thing a cheater wants is to think that they have out- witted the S. She will feel horrible and will worry that you might know.
