I totally agree with Wolf. Don't spy.
If you feel the need to spy, there is either something wrong in the relationship or with you. Yes, someone should have checked on Bernie Madoff, but that's business, not personal. If that's how you view your personal relationships, then that's not good. (Also, I think the unbelievably large investment returns Madoff's investors were supposedly seeing should have raised some eyebrows, but that's another topic).
Yes, you can ask where he's been or who he's been with without badgering. You can look out for signs of cheating or lying, basic consistency, but snooping goes too far. The problem with it is that when you find nothing, that gives you a pay-off. It's like, okay, it's clean, so I can relax now. Unfortunately, that relaxed feeling doesn't last. It becomes like a person with OCD checking the stove. Each time they are obsessing over the stove being on (even if they checked it five minutes ago), they can't stop thinking about it until they check it again. You can't let yourself fall into that pattern. It will destroy both you and your relationship.
If he does cheat, then you need to require complete transparency from him, but until then, he's entitled to your trust. Now you just have to earn his back. It may take time, but since it was just one slip, it probably won't take much time for him to forgive you.