It's an ugly road you're going to travel down.
Now that that's been said first off I say just drop her. Ghost her if you can. Rip it off like a bandaid.
I'll also throw some other advice in here if you're dead set on getting into her business.
Getting into her phone would be tricky. Old school phones it was easy peasy. But a lot of smart phones and stuff can sync up to PC, TV, Playstations or whatever. If that's been done there's a backdoor.
Then there's the whole PC rout. If it's a desktop PC just grab a usb keylogger from thinkgeek. Unplug keyboard,plug keyboard into keylogger, plug it all in. In a few hours/days you have access to anything they had access to.
FB is tricky nowadays because it keeps track of devices used. And depending on how paranoid they are they may have their FB set to lock down if an unknown device attempts to log into it. But generally that's where a trail would start.
Now before people go ripping me a new asshole over this post I did this for a living for a few years on the side. I've seen some shit. Worst case scenario kinda shit.
OP I'm just going to tell you right now you should probably just step back and call things off. You have enough red flags there to tell you everything you think you need to know. The whole "what if" I'm wrong and I'm making something out of nothing. I'm telling you there's a 99.9% chance you're right. You're going to do nothing but rip yourself up over this whole thing.
So
1 Drop her.
2 Get yourself a chick who's into other chicks as well.
3 Live pretty happily and get a nice 3way in every now and then.