In my case it was a series of comments and cues that led me to ask her if something was going on.
There was the conversation where she asked when would she ever find time to cheat and I replied that if she really wanted to she would find the time.
There were subtle changes in attitude towards me, she became less tolerant of my mistakes and foibles.
There were visible bruises on her upper arms.
She sometimes got back from evening classes later than expected.
There were changes in her, she was more anxious and, when in the house, would spend more time alone in the kitchen.
There was less sex, she appeared to be losing interest.
There was a bottle of alcohol hidden in her piano.
When I challenged her she admitted to fucking a co-worker she had been going to evening classes with. During their lunch breaks she had been bringing him home and fucking in our bed and, while I was away on business, they had spent a night in a hotel. Apparently she always was on top riding his cock hence the bruises on her upper arms where he gripped then.