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!

used to express cynicism or doubt about an assertion. informal
See also: could, fool, have
Farlex Partner Idioms Dictionary

you could have ˌfooled ˈme!

(informal) used for expressing your surprise about a statement, claim, etc: ‘He’s quite intelligent, you know.’ ‘You could have fooled me! I’ve never heard him say anything intelligent at all.’
See also: could, fool, have
Farlex Partner Idioms Dictionary
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