Nan,
Trusting your husband is very important. If you don't trust him, you won't be totally happy with him. You should have a heart-to -heart talk with him and let him know that you have doubts about his fidelity. Just let him know that you care for him a lot and that you love him, and that you want total honesty from him.
Otherwise, it's hard to tell if he's actually cheating unless you can afford to hire a Private Investigator.