How do you spot a cheater ?

All of these posts about cheating men, women, etc makes me wonder if there is a way to differentiate between a cheater and a loyal person.

Do you have any tricks up your sleeve that you use to determine a cheater?

I had a public speaker come to my school back in high school and tell us that she knew her husband wasn't a cheater because he and her promised to not have sex until marriage. To her this proved that he can control himself enough to wait for her until marriage so he would be able to control himself if there were ever a situation that could be tempting.

How do you spot a cheater ?