wasn't born yesterday

I/he/she wasn't born yesterday! (something that you say in order to tell someone that a person is not stupid and cannot be easily deceived) -- не лыком шит; не вчера родился; не первый раз замужем;

Example 1: You'd better think of a better excuse about the dent in my car. I wasn't born yesterday, you know! (CIDI)

Example 2: You can't expect your mother to believe that - she wasn't born yesterday!(CIDI)

see also

[nobody's fool]
[не лыком шит]
fall off the turnip truck (a derogatory expression used to describe someone who is not very smart or who is dumb)