take (one) for a fool
To presume or believe that one is very stupid or foolish. I'd advise you not to take her for a fool. She may play dumb, but she's very crafty.
take (one) for an idiot
To presume or believe that one is very stupid or foolish. I'd advise you not to take her for an idiot. She may play dumb, but she's very crafty. Do you take me for an idiot? There's no way I'm falling for that again.
take someone for an idiot and take someone for a fool
to assume that someone is stupid. I wouldn't do anything like that! Do you take me for an idiot? I don't take you for a fool. I think you're very clever.