I have 2 ideas in mind.
1. Like what above said, make a fake account. Accept & test him. See what he doing with his another account. Maybe you could even try to make him fall for the fake account if he's really doing something wrong.
2. You can just talk to him, tell him what you know and just ask for an explanation. If you feel that the marriage can be saved, talk it out. :)
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Founder of Yellow Ducks and Me