Before we discuss tactics, Brknhrt, we should probably talk a little about why you'd be doing it, and what you'd be hoping to accomplish.
In general, I'm in favor of gathering reliable intel ("snooping") vs. the guessing game/sticking-their-head-in-the-sand that most people stumble their way through. But you have to:
a) be doing it for the right reasons; and
b) be emotionally able to handle what you learn, and be willing to deal with the fallout.
What ARE you hoping to accomplish, and how DO you think you'd be able to handle it, emotionally, if you found out the worst?
Keep in mind, too, on this latter point, that unless you hire some professional, who I'm presuming is somewhat trained in how to break it to you gently, you'd be seeing/hearing RAW INTEL (for me, it was actually seeing my beautiful wife and mother of our four children get out of her car, and get into OM's beat-up old truck, and drive off to his house to have sex; and listening to recordings of her having sex with him and him telling what a "little slut" she is, and her LIKING it).
I say these last things not for shock value or to in any way denigrate my wife, with whom I'm now almost fully reconciled, but as a way of warning to you.
