So instead of reviewing the entire book, which sympathizes and explains cheaters and cheating behavior, I've just skipped to the most pertinent chapter.
Cheaters have a rehearsed lie if they ever get caught. Denying, having a good story all memorized so that it comes out sincere.
Because cheaters can become so good at lying, social media and apps can be a way to find out. Pretend your phone is dead, and ask to use your partner's phone. A phone can be the key. Google or Apple accounts that automatically connect everything can hold evidence of cheating.
Social media makes it easy to get caught. Maybe a cheater gets a picture taken with his affair partner. Maybe the cheater is following his affair partner on Instagram.
A good past reputation will give a cheater benefit of the doubt. The cheater needs to make sure his partner is happy so she is more trusting, less suspicious.
Paying a bill can leave a trail. Cards, payment services leave records. Using a card more, or a large amount charged. Maybe even a secret credit card his partner doesn't know about.
Cheating during the day is less suspicious than cheating at night. If a job has random hours, changing schedule, travel as part of work, fake work trips.
If a cheater's routine suddenly changes, like more frequent showers, it may indicate cheating.
A cheater may have friends who enable him and back up his lies.
When a woman suspects a man of cheating, he almost always is. Save the energy and dump, divorce as needed. Your best friend is your natural instincts--they cannot lie to you, but men gleefully will.