Most Helpful Guy
If you are dating a guy, and your communication is in such a sad state that you don't know what he's doing (the basics, such as, his work schedule, and if he goes to his buddy's on Thursdays for BBQ night or whatever), then your relationship already has SERIOUS problems, even if he's not cheating.
I'm not saying that you need to keep minute-by-minute tabs on him, or that he should have to check in with you every 30 minutes, but guys who don't have anything to hide generally will say "Hey, me and the guys are going to Happy Hour on Wednesday, so I won't be home until after 10pm." They won't be secretive and try to keep that away from you, and they'll have you meet those friends and probably do things together with them on other nights.
Communication is at the very foundation of a relationship, and if you aren't communicating, it's simply not possible for the relationship to be healthy. And if you ARE communicating, it's very, very difficult to hide cheating, because most people feel guilty and usually pull back, then overcompensate. It's an easy pattern to spot.