Have you been cheated on? How did you find out?

Cathy
Have you been cheated on? How did you find out? 
My first boyfriend cheated on me with my best friend. I found out four years after I had dumped him for sexually assaulting me (yeah! Dodged a HUGE bullet there) 
A mutual friend mentioned something about him cheating, and I asked what she meant. Her response was "I thought you knew he cheated?" - I was the last to know apparently, and she had assumed I didn't speak about it because I didn't want to, and no one knew the real reason I dumped him. It took me years to admit that to anyone out of shame/embarrassment. 
And the 'friend' that he cheated with, I confronted her about it and asked why she didn't tell me. Apparently so much time had passed she 'forgot' to mention it.... !!