3 you could have fooled me spoken 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 - Definition from Longman English Dictionary Online
Well, the wife (presumably) is not disbelieving that he's a human.
She's saying, "You're doing a good job of behaving like an animal, so I could have almost been fooled that you weren't human."