To answer your question you need to listen carefully to their responses.
People usually speak the truth, but rarely do we hear it. Usually, we put our own interpretations on what they are saying.
For example. If you ask: Did you have any physical contact with that woman? and he answers something like: I swear I never laid a hand on her. As God is my witness I never touched her. Basically, if he says anything other than a direct No, I didn't touch her. The relationship was not physical then it SOUNDS like a denial but it IS NOT a denial.
If your question is answered by a question eg. Why would you even ask that? or a statement like You're crazy. I would never cheat on you. etc then again, it IS NOT a denial. It does not prove that he did touch this woman but it is indicative that he has something to hide.