you could have fooled me

(you) could have fooled me

Really? I thought the opposite was true. A: "This neighborhood has really improved in recent years—it's quite popular now." B: "Wow. With all the boarded-up buildings, you could have fooled me." Maria is shy? Could have fooled me. She was talking all night.
See also: could, fool, have
Farlex Dictionary of Idioms.

you could have fooled me

People say you could have fooled me to show that it does not seem to you that something is true. `I came here to apologize.' — `You could have fooled me.' Car makers are supposed to be bringing their prices down. Well, you could have fooled me.
See also: could, fool, have
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • (you) could have fooled me
  • could have fooled me
  • you could have fooled me!
  • coulda
  • (you) coulda fooled me
  • Who can say?
  • Who can tell?
  • at the back of beyond
  • Who's to say?
  • somewhere to hang hat