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Question

nobody's fool : a sensible and wise person who is not easily deceived.

ex:  Mary is nobody's fool. She watches out for people who might try to cheat her.

Anne may seem as though she's not very bright, but she's nobody's fool.

Comments

nice +1

Also:

be no fool (also be nobody's fool)› to not be stupid or easily deceived:I notice Ed didn't offer to pay for her - he's no fool.http://goo.gl/mIOWhH
 

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I'm not falling for that one!

said when you recognize a trick and refuse to be deceived by it

"Lend me a fiver and I'll buy you a drink." "Oh no, I'm not falling for that one."

Answer 1

(there are) no flies on sb. (informal) :

If you say there are no flies on someone, you mean that they cannot easily be deceived

example: 

She may be new to this field, but there are no flies on her

Answer 2

Be taken in

He's not taken in easily

pull a fast one:

After catching her five-year-old son Lucas trying to pull a fast one, his mother demanded, “Do you think I have idiot written on my forehead?”
Lucas answered, “I don’t know. I can’t read.”