Wiktionary
could have fooled me
phr. (alternative form of you could have fooled me English)
Usage examples of "could have fooled me".
But that was nonsense-- if an Anderson could have fooled me, another would have done, it long before.
Yes, he could have fooled me, especially since I only saw him once a month and didn't have a chance to observe the mood swings and personality changes that would have been more evident if we'd had weekly contact.