There's no way to discover a partner has been unfaithful that won't sting. But catching them in the physical act of cheating seems like possibly the most painful way to find out the truth. Nothing burns the eyes like witnessing someone you love in the arms of someone who is not you. But there is a bonus: it sure makes it tough for them to lie or justify their way out of it!
1.) From corbin1:
Few years back I lived with a buddy in a large apartment and we always had parties. I was flying back from Florida and wound up getting home when the party was "winding down". Everyone was under blankets on the living room floor. Saw my "buddy" with what was obviously a figure halfway down his sleeping bag doing SOMETHING to him. He gave me a wide-eyed look and I cracked a smile at him until my girlfriend of two year's head popped out from the sleeping bag. I was in shock for a few seconds, and Tyler went to talk. I cut him off and said "you can keep her". Drove across town to my moms that night, dun cried like a bitch too.