You will never really know for sure unless you actually catch them in the act. The only advice I can really give you is that you just have to trust her or end it. You said that she lies to you; it’s good that she tells you when she lies but at the same time, you shouldn’t waste your time w/ someone that lies to you. Relationships are only successful with good communication and honesty. If the honesty isn’t there, you need to really ask yourself if this is someone you want to be with in the first place. You are in tough situation and I know exactly how you are feeling. There are some things that you definitely should do and should not do. Don’t accuse her of things; this will only cause more tension in the relationship and create more problems, not to mention stress you out even more. Also don’t think about things too much, you will start convincing yourself that she is doing things that she might not be. What you should do, just be you, after all, she’s with you for a reason, Just be yourself and show her through your actions/words why she got with you in the first place. If you start getting jealous or make false accusations it will only drive her away, you want to do things that will encourage her that you need to be the only guy. There are other do’s and don’ts but I’m at work and I need to get back to it, but just try to keep the communication open w/ her and figure out if you want to settle for someone who lies, cuz in the end, you need to do what’s best for you.