Cheaters can have several tell-tale signs.
1. Deviation from the norm / schedule (working late eh?)
2. More cell phone calls and reluctance to answer around you (what are they hiding?)
3. Secret / questionable content on their computer. Are they closing screens when you enter or always deleting history?
4. Change in appearance for no reason (all of a sudden, they're doing their hair to go to the mall or dressing up to go grocery shopping)
5. Decrease in sexual appetite - where else are they getting some?
6. They get defensive when you ask questions, even if they're innocent (the guilt gets to them)
7. The passenger seat on the car has changed even though they claim they haven't driven with anyone.
8. Money troubles - do they have less money to go out? Maybe they're spending it going out with someone else.
9. They stop wearing their ring or anything you gave them.
Ofcourse, all of this should be taken with a pinch of salt - nothing works better than sitting down and talking about your concerns.
Good luck to you all and I hope you never have to deal with a cheater!