They're onto me

[aleCcowaN]

I just took a glance at a "caja boba" (TV set) near the place I am and knew they're broadcasting "Liar, Liar" again. I remembered the gagreel in the credits section and a funny joke at Jim Carrey's hamming expense.

The dialog is:

Lawyer: - Now let's see. "Weight: 105" ?
Fletcher Reed (Jim Carrey): - Yeah ! ln your bra.
Lawyer: - Your Honor, l object !
F.R.: - You would !
- Overactor !
- Jezebel !

[Everybody laugh]
[ Applause ]

L (pointing a finger behind the scenes)- He put me up to it.
Jim Carrey (laughing):
-"Yeah, sure he did.
Oh, no.
They're onto me."

[All Laughing]

I always doubt about the meaning of "they're onto me". Most of the time I relate it with the Argentine expression "me tienen calado" meaning "they know me well/they discovered my trick, defect or weakness", some times I think it as "la/s tienen conmigo/contra mí", and now and then I imagine it being "se la toman conmigo/contra mí".

What do you think? How can be translated all three expressions?

[Please, correct "a discreción"]