You are separated, how? You moved out? She moved out? It's an In House separation?
How old are the two of you? Any kids? How long have you been married? Together?
Is an affair a deal breaker for you? Does it make a difference if it is emotional or physical? Many people think an A is a deal breaker and then find out that they are able to forgive an A.
I guess you consider it IS a deal breaker, as you are looking for proof. But... you can't find any... and it's driving you crazy...
How much time and effort do you want to put in to finding the "proof"? What if, after spending all that time snooping... weeks and weeks and weeks... or hiring a PI to get the proof... that there is none... what then?
You can assume there is and move forward accordingly...
or you can assume there is NOT... and move forward accordingly...
and in many cases... moving forward "accordingly" is the same thing.
Have you picked up and read Divorce Remedy by Michelle Weiner Davis? If not, please do so. Most of what we discuss and the work we refer to is based on the book. The book should be your FIRST reference and this board is a sounding board and support effort.