I don't have physical proof of an affair on her part but there are too many signs/signals to dismiss that it didn't happen.

I've brought it up with her and she denies it - and apparently that's standard behavior (deny, deny and deny some more).

I don't bust her hard on it but I told her that I need her to be honest with me. I won't label her as a cheater and I can forgive her but she has to come clean with me on this. I told her it doesn't define her as a person, I can understand that it was a bad decision on her part and people do make bad decisions.

Everytime I bring this up, she denies it.
(I don't make it a point to bring it up every 5min. of every day but sometimes a conversation will go into a specific direction that opens up to this type of question).

Here's the catch, a day or two later after I bring this topic up, she does something nice for me. It's almost like a guilty reaction, I'm almost tempted to start logging the reactions based on my inquiries into this - it's damn near scientific at this point. Do this, observe this reaction, Repeat.