After spending time with someone for a number of years (or months), you know someone's behavior and patterns. You know how they will act and/or react to situations, and when they fall out of the pattern that you're used to, there has to be an explanation — and if not cheating, then something real and tangible. A sudden change in pattern is what you're looking for: going into the restroom for 20-30 minutes with his phone to "read the news" when he never did before... a new password on his phone or being protective of it when you are around... taking phone calls in his car before coming in the house... if these behaviors aren't part of his normal routine, then they are all red flags.