How do I check to see if my catalytic converter is good?

I took my car to the shop, because all of the sudden it does not want to run right. When I start it it stalls unless I keep foot on the gas and brake to get in gear. When I step on the gas it hesitates like a lot of gas and then no gas even though my foot is sometimes to the floor. Took it to the shop and they said it was catalytic converter. I took it off and it does same thing. Changed the fuel filter, and the distributor cap and rotor. Please help!