From Longman Dictionary of Contemporary Englishyou could have fooled meyou could have fooled mespokenBELIEVE used to show that you do not believe what someone has told you ‘Look, we’re doing our best to fix it.’ ‘Well, you could have fooled me.’ → fool
Examples from the Corpus
you could have fooled me• "Look, we're doing our best to fix it." "Well, you could have fooled me." 