Do you snoop through your partner's things? Have you ever read their diray / email / letters? Only one time, EVER, in a very happy relationship that continues today, as it has since 1981. To make a long story short, I had very good reason to believe that particular people were trying to manipulate my dearest one. I snooped on an email. I found exactly what I expected to find: a very manipulate former roommate and (now) former friend who was seriously attempting to interfere with our relationship and interject herself in ways that only caused trouble. To my relief, I also found my dearest partner in life resisted and consistently objected to the intrusions. But it was still EXTREMELY unpleasant to confront her with what our former friends were trying to do. She really did not want to believe they were being truly malicious. The good move that made all the difference was simply not trusting my own opinion alone. By this time, my oldest child was 16 or 17 years old, and I asked for his opinion in the entire matter. He helped intervene, and it was more dramatic than I can possibly convey. But we're a closer family because of it. Your milage may vary.
I would say never look, not only for the invasion of privacy, but would you be able to ever deal with what you found out? Sometimes it is easier on the heart and the relationship to never know. If you really love someone, to just remove the temptation to look, don't do it until there is a definite in your face clue that something is wrong. If you look and end up hating each other because of the arguments, no matter if there was infidelity or not, then you both have lost. If you don't look and they are unfaithful then it will come out in the end, and then check up. If you don't look and they are true, then you can still look each other in the eye each night. If you look and they are faithful then you have betrayed their trust. If you look and they are cheating then how do you deal with that knowledge, wouldn't it be better to have never found out? I learnt this the hard way, I believed what others were saying about my girl, and ended up mistrusting her, looking through everything and doubting and questioning each and every second of our relationship. It sounds whiney but I wish I had just trusted her no matter what because now I know that although we love each other we have so much mistrust a simple conversation can be a tinderbox to huge arguments.
If someone is doing it then it's pretty safe to say the relationship sucks. We go out on a limb in relationships and it's about letting down the walls we put up from being hurt and lied to before. So it has to be perfect trust and yes many times it ends up where if we did pry we would have found out sooner but a relationship built on this never lasts. My own experience has been the people who are snooping are doing it because they are not being honest and are then coming from fear. They go out and cheat and then worry afterwards what the OTHER person is doing knowing deep down what we do on one side happens on the other. I had just had someone blow me off in a relationship and that day an old love showed up and we hung out. Nothing sexual since at the time I was still in love with this other person. But now I see it as she manifasted this person coming back in my life since she has been far from honest with me. So it worked out in my case.