1. The Usual Suspects
Check his clothes (specifically his shirts) for lipstick marks or any other unusual marks that could indicate sexual activity. Scratch marks or another woman’s scent are a major indicator that your significant other is getting it on with someone else.
2. He starts working harder all of a sudden.
He starts staying at work after hours more often and might start going away for business trips frequently. This, along with a few other signs such as expensive meals, hotels, and other expenses that don’t include you, is a pretty strong sign that something is up.