Okay, first hit: "Dad always said that you can always tell an idiot... but you can't tell him much." Works for my point just as well as "bigot", I suppose.
Hint: "You can always tell a XXXXX, but you can't tell him much" is a stock construction. It most certainly is not used to indicate that XXXXX has superior powers of reasoning.
Hint: "You can always tell a XXXXX, but you can't tell him much" is a stock construction. It most certainly is not used to indicate that XXXXX has superior powers of reasoning.