I'm sorry you're reeling. This is rough stuff, to be sure.
I'd like to get you to just prepare your heart that this has most likely gone physical, and probably not just recently. Most men do not stick around in EAs for 8 months without "a little somethin'" and at some point, if she hasn't already, your wife is going to give in to that if she's still not committed to the marriage.
Rather than make yourself crazy, I would encourage you to just operate from a position of she's already having a PA, and act accordingly. Because A), all the reasons above, and B) EAs -- esp. long-term ones -- are stronger for a woman than a PA anyway, in most cases.
If you REALLY have to know, I guess you could just hire a PI.
