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.
(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.