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 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.