It depends...what is making you supspicious? I suspected something with my hubby, and actually busted him...long story, but if you tell me what you think is going on and why you think it, I will give you some pointers.
If you have a guy feeling he is cheating he probably is. I know that sounds harsh but most of the time us girls are right. You have to find out first that he is because you can't accuse because that will make you look like a psycho. Pay more attention to what he's doing and the truth will come. But don't wait too long!