There are ways to see his emails and track his online activity that are undetectable. Works well if he is somewhat computer challenged. This is how I found out my ex was cheating.

To my knowledge Jerk never cheated on me, but I suspected it at times. At the end of the day it doesn't really matter...I obviously didn't trust him and no good relationship can exist without trust.

I'm soon to be divorced but been separated for 4yrs. When we were living under the same roof my ex tried to put a key logger program on the computer to see what I was doing, "guess he felt guilty about what he was doing" needless to say he left this program open one time and busted his self. This is just one of the many times I caught him cheating.