This is always Rule One. We too often forget it (as just assumed) when we say Rule One is GAL and take care of yourself.
Look ... snoop if you need legal evidence that will work to your advantage in a rough D.
Otherwise, you will find things that don't mean anything and make yourself sick analyzing them wrong. Or, you will find something hurtful, and focus on pain you don't need right now. Bruce, you are invading her privacy. Please don't. If you want any chance at a future with her, respect her privacy.
That said, it is ok to assume. Assume that if this really is MLC, there probably will be someone at some time. Part of the transition is the MLCer processing the things that are obstructing their passage to their next life stage. That may be the mourning of their perceived loss of youth. They often feel drawn to live their early years over, have a fling, be 17. Your girl is a prime age for this kind of crisis. Add her past depression and low self esteem, you got yourself a Gold Card carrying MLCer.
Sorry
But what I want to say is do yourself a favor man. No snooping. This subject is usually the basis of threads over in Newcomers before folks learn about the possibility of mlc. By the time we got here, we snooped, got burned, got caught, made the whole thing worse, etc. Stop now before it is too late. Why is she going to those web sites, why does ... end phone calls, change checking accounts, get hot new clothes, levitate over the bed and spew green pea soup ....
It is what it is. You need to give her time and space while you do the same for yourself. Be kind to yourself. The only good that can come from snooping is the kind that comes after you have already made up your mind to get a L, file, and destroy her in court at all costs. Is that where you are, or where you want to be?