How do u know if your cadalladic converter is plugged?

how do u know if your cadalladic converter is plugged? i have a 1997 pontiac grand prix gtp and when i have the car ideling and i hit the gas the car bogs down when the rpms reach 4. i dont know if its the turbo kicking on or what? but when im driving the car and punch the gas it does not do it. can anyone help?