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.

Inf. I would have thought otherwise.; I would have thought the opposite. Henry: Did you know that this land is among the most productive in the entire state? Jane: You could have fooled me. It looks quite barren. John: I really do like Mary. Andy: Could have fooled me. You treat her rather badly sometimes.
See also: could, fool, have
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • (you) could have fooled me
  • you could have fooled me
  • you could have fooled me!
  • (you) coulda fooled me
  • coulda
  • Who can say?
  • Who can tell?
  • at the back of beyond
  • Who's to say?
  • somewhere/some place to hang (up) (one's) hat