I found out that I had been cuckoled when I went to my girlfriends apartment one Sunday morning (we were not yet living together). She was in her bathrobe, and not being a very bright 22 year old, I thought she just wanted to be ready for sex when in fact she had just been fucked by her previous boyfriend who had left the apartment a short time before I got there. I didn't notice until later that there were two cereal bowls on the drainboard and that there was a wet spot on the bed.
I did notice that she was very wet and had an unusual taste. After a short time her vagina let go of a gob of slime, and I stopped sucking. She put her hand over her cunt and went into the bathroom. When she came out she didn't say anything, and she let me fuck her.
When we went back to the kitchen she noticed the two bowls and assumed that I had figured out what she had been up to. She said that she was sorry, and that she had not intended to have sex with her ex. To my surprise, I told her that it was OK and that she should continue to fuck him.