I'm single and looking, I've had married men hit on me... I always ask to be added to their FB friends list. Sometimes they lie and say they don't have one, but when I search their name, & find their photo, their married 'relationship status' is clearly visible.
Others add me and act shocked when I call them out on the wedding photos where he is obviously the groom and his wife is making lovey-dovey comments on all his posts. I contact the wives if I can before I call them out though. I message a copy of what they said to me to their wives. and tell them exactly what website he is using to cheat with, and all the gory details of his profile.
I had one wife reply back saying thanks for letting her know. He's cheated several times before. She even had the divorce papers ready for the 'next time', and she was going to file them the next day. She was pissed, I'm sure he barely got away with the shirt on his back.