(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
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.