As a person with trust issues, I would be struggling with this situation myself. My moral values are very stringent when it comes to cheating and not only do I not tolerate it, but struggle to associate with those who do. My concern would be that your SO considers cheating okay as she has been participating in a relationship in which her partner has been cheating. This would set a red flag in my head too. It is great she hasn't lied to you, but obviously there are some major trust issues that need to be faced here.
If you are to the point that you are considering checking her phone, or wonder where she has gone every time she isn't home, some sort of intervention needs to occur. Immediately. In my mind, cheating and lying do so much more damage in a poly relationship. In these sort of situations, everyone has to feel secure and open or things tend to get ugly really quickly.
I wish I could give you better advice, I just want you to know someone understands your thought process.