Well - another consideration if you are not SURE what's going on, is to find out the truth. Snooping can be a negative thing if it gets you all worked up. On the other hand, if you really just need to know the truth so that you can plan your life accordingly (i.e., if him resuming the affair and lying to you about it is a deal-breaker for you that would make you want to separate or confront him or do something differently than you are now) then gathering evidence so he can't gaslight you may be in order.
Standing is NOT the same thing as being a co-dependent doormat.
