Been there, done that. That is how I found out H was actually living with OW, who at that time was supposedly just a FF and not the OW yet.
Anyway, what I can tell from my own experience is that when you want to spy it shows that you have not let go yet. When I lay out the cold hard facts for me that helps me to get anti-H and gives me the who gives a "leap" what he is doing.
My thoughts go..."H lies all the time, he decieves me, I can't trust him. I have tried over and over again and he continues to lie. H had an affair. I'm not even 100% certain if it is over yet. H still works with OW so what is the point anyway. I WILL NOT be M to someone who continues to have some sort of R with OW. I deserve much more."
There you have it. When I think along these lines I am disgusted with H's behavior. I detach 100% and prefer to NOT be with him.