I had that "gut feeling" that something was up between my exH and his wistress but for months he told me it was me, I was jealous, I had trust issues, etc.
Then one day I got a letter from a very, VERY expensive hotel we had done some computer work for but which "we" had never been able to afford to go to. The letter had an intimate item in it, and said "When you were here the other night, you left this in the room." Needless to say, I had not been there, so I went to the hotel, asked if I could see the registry and saw "Mr. and Mrs. <our last name>" on such and such date. Then I went to the computer room and looked through the security video for that date (hey being the PC person has certain benefits) and got a copy of him and her going INTO the hotel at night, signing in, and coming OUT OF the hotel the next morning with smooches and whatnot.
I went to our office, showed him the video, and slapped him across the face so hard that his glasses broke. He asked me "How did you do that?" I said..."Do what?" He said "Doctor that video to make it look like I cheated on you."

Then he called the police on me for physically assaulting him.
I would have reconciled with him too if he had even once admitted what he had done and made an attempt to work on his own issues. He refused to address any of them, so I refused to reconcile ... and the rest is history.
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For my own infidelity, I was discovered when my Dear Hubby got the OM's email address and emailed him a letter that effectively said, "I am her husband and I do not intend to let this marriage go without resistance. She's my wife and I intend to honor my vow" and frankly that scared the OM away.
God...it's so humiliating to even remember how I acted--plus I KNEW BETTER!! BUT it just goes to show that infidelity is insidious and can happen to anyone if the mix is right. I did it and thank God Dear Hubby had the balls to stand up or I may not be here in my right mind today!