A story I heard, probably a myth, or still made up, mentioned a woman suspecting this but not wanting to believe it about a guy, so she took a colored gelatin solution, coated the bottom of his shaft and balls with it, and then he 'went out.' After coming back that night/next morning or whatever, she supposedly was able to figure out that by the way it'd been wiped away, he'd been fooling around.

Personally I think just body sweat would have caused that last part to happen. As it is, neither of the two people I'm thinking of are trustworthy in any way, shape or form anyhow - one would think they would have been perfect for each other....
Supposedly he wandered back into the arms of someone he'd dated before marrying this other girl, but I'm getting far away from the subject here