what are the warning signs? Do you know them?

Loss of interest or concern. Not wanting to share time anymore together. Not communicating anymore. Not around, excuses why. Being secretive. Phone calls that hang up when the wife answers. Smells on his clothes.

A new interest in how they look, new names of people as friends or more alleged time with friends that you know. Secrets or strange hours. Getting caught in small lies (usually means there's bigger ones). I spent a year helping my friend catch her husband cheating and even though he had so many different signs, she needed to catch him herself to believe it

I think a noticeable change in behavior is something to take note of. He may become more distant, or he may actually become more attentive if he feels guilty. If there's a sudden interest in "trying new things" that would raise a red flag. Not that that's always a bad thing, but if all of a sudden there is a new interest, I would wonder why. He may not be where he's supposed to be at odd times. He may get lost in thought more easily. Mine actually started talking about one of the women he had an affair with as if he just couldn't help himself. He didn't come right out and tell me, but she was the topic of conversation so much I started wondering what the deal was.