You can't be sure, inthedark, you have to decide whether you want to trust your girl and see how it goes or assume something bad is going to happen and break up with her. If you decide to trust her, you have to do it all the way, assume she will stay in relationship with you and not lie or cheat. If she does, then you will break up with her but why assume the bad (that she IS lying and cheating) first? There are only so many ways the situation can go (she is lying and cheating and (1) you learn of it (2) you don't learn of it (3) you learn of it later or she is/does not lie and cheat but (4) you accuse her of doing so or (5) you trust her). Only five is what you want and the only way you can get that is to start from there and skip the rest until who she is and what she does becomes obvious and either rewards your trust or hurts you.