There are only two reasons why people lie by omission or deceive you.
1. To avoid the pain or repercussions of dealing with the truth. This includes causing someone else pain and dealing with their anger, hurt, and disappointment.
2. To keep the thrill alive. Whatever they are doing behind your back they don't want to stop. Therefore they lie to avoid being pressured to stop.
Ultimately it comes down to whether or not you trust (your) gut instincts. You can ask her point blank about something and see what type of response you get.
Trusting someone does not mean you can't ask them questions. Trusting someone means you believe they will give you (honest) answers to your questions. A person who is "in love" with you wants to assure you that you have nothing to worry about.
Anyone who answers your question with a question, gives you a short quick answer to shut you up, becomes loud or indignant, starts calling you "paranoid", accuses you of being "insecure" or takes offense to you seeking some understanding/clarification is probably not concerned with what you think. There goal is to (shame) you or (talk down to you) for asking questions. It's an attempt to deflect answering your question and deter you from following up.