I've been cheated on, and unfortunately he managed to keep it out of the radar of everyone we knew, but if they had known, I'd EXPECT to be told.
I would probably do like a couple others suggested and walk right up to him. Except I'd probably first take a pic on my cellphone and then throw in the line "So I hope you're planning to tell your wife about his so I don't have to....." Best part would be if the chick he was kissing didn't know he was married.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi