I once went out with a guy called Matthew. I really liked him. But we broke up.
My best friend Anne was dating Matt's friend Mark. Mark had gone to England for work and had sent Anne a postcard.
Anne and I were house sitting and I'd collected the mail, giving Anne the postcard. She had a quick look over it, not reading it then handed it to me, saying I could read it if I wanted.
I read it through, and got to one sentence:
"Matt was cheating on Sarah"
Nice way to find out.