How do I get my husband to admit if he's cheating?

My husband cheated on me one time (that I know of) last spring with some tramp he'd known for a week. After a short separation we decided to give things another try. For the past year he has appeared to be bending over backwards to show me he's sorry. Now he is deployed...again...and I am having a really hard time trusting him. I have nagged him for months and he continues to swear that nothing is going on. I don't know if I have a gut feeling that he is cheating or if this is leftover from the last time he did.