When I "broke into" my H's MySpace account, I did it right in front of him.
I did the "Forgot My Password" thing and had it e-mailed...shockingly, it was e-mailed to my H's phone e-mail (oh, but it wasn't HIM). I have access to that since I pay the bills online. When I log onto Sprint, I have access to check the e-mail. I got the password and entered it into MySpace and there he was - single, no less. It was also ironic that the MySpace screenname was nearly identical to the LavaLife profile that also wasn't him.
He stood right there and watched while I did it, turning paler and paler with each step.
I will say this, OP.....be prepared. If you think something you see would devastate you, you might be better off not doing it or just waiting a bit. By the time I discovered my H's internet activities, I was already over it. My 3rd DD was born in May and he wasn't allowed at the hospital when I delivered because I was done with him. I discovered his internet fun in July. So, I had a period of being over him before I found what I found. When I saw him messaging women that he looked at their pictures and they were hot and asking women if they were single and telling one woman who listed herself as divorced that her ex must have been out of his mind because she's beautiful, it didn't even bother me because I was already done with him (I was actually more annoyed that his chatline was charged to our phone bill - like I should have to pay for him to chat up other women).
If you're not prepared to see something along those lines, don't do it. You really have to mentally prepare yourself to find something shocking and upsetting.