you could've fooled me

You could've fooled me. {infml} — Так я и поверил! Ищи дурака — так я и поверю! ~ Ну кого ты пытаешься обмануть? ~ Ври, да не завирайся. || Не может быть! (удивл.)

Used to say that you do not believe what someone says about something that you saw or experienced yourself, or that you are very surprised by what they have said

Example 1: 'I’m doing my best, honestly.' 'You could’ve fooled me!'

Example 2: 'No, I wasn't angry, I was just a little surprised.' 'Really? You could've fooled me.' (thefreedictionary.com)

Example 3: 'I’ve been offered a part in a Hollywood movie. Isn’t it great?' 'You could've fooled me. That’s unbelievable!'

For more expressions of disbelief see:
[так я и поверил]