Have someone pegged - When there is someone who pretends to be very good, very charming or anything like that, or even does not pretend to be so, but what matters is you have a doubt on that person and in fact you want to know what kind of person he/she is, you will try to make out. When you try to make out what kind of person he/she really is, you say that you want to have him pegged.
1. I really doubt on that person, I want to have him pegged.
2. He thinks I am a stupid fellow that I will invest in his stupid scheme, I have got him pegged.
3. Why don't you try to have him pegged, even I have a doubt on that mischief?