It's entirely possible your GF doesn't have trust issues, she's just nosy and wants to know what you talk to people about. It's worse if she thinks you might be cheating. Usually it's the people who are always suspicious of their spouses who actually do the cheating.
As to finding out when she's doing it, when you close your phone and set it down, go to an app that doesn't have multi tasking of some sort. As in, any app that when you reopen it doesn't jump back to the state or place it was in. That will be a dead giveaway.
Also, just out a lock on your texting app. If she's claiming she isn't doing it, then how will she know you've locked it? When she brings it up, finally, tell her flat out that you dislike her going through your private conversations with friends behind your back. But tell her you'll let her read all of your texts anytime she asks.
I also like the fake SMS idea with a friend. A better way to do it would be to use HER number, naming her something else obviously sexual so she doesn't realize they came from her phone at first glance. Send yourself a couple of flirty texts from her phone, and then end it with "I know you're reading this <GF's name>..." combined with careful selection of both where you closed your phone and where your SMS app was at when you last got in it it should be easy to know if she's been snooping.
As to finding out when she's doing it, when you close your phone and set it down, go to an app that doesn't have multi tasking of some sort. As in, any app that when you reopen it doesn't jump back to the state or place it was in. That will be a dead giveaway.
Also, just out a lock on your texting app. If she's claiming she isn't doing it, then how will she know you've locked it? When she brings it up, finally, tell her flat out that you dislike her going through your private conversations with friends behind your back. But tell her you'll let her read all of your texts anytime she asks.
I also like the fake SMS idea with a friend. A better way to do it would be to use HER number, naming her something else obviously sexual so she doesn't realize they came from her phone at first glance. Send yourself a couple of flirty texts from her phone, and then end it with "I know you're reading this <GF's name>..." combined with careful selection of both where you closed your phone and where your SMS app was at when you last got in it it should be easy to know if she's been snooping.