An effective way to see if the cat is doing its job is by monitoring the 2nd o2 sensor (the one after the cat, it should be reading a stable voltage between 0.7v and 0.9v whilst driving at 50% throttle, indicating no unburnt fuel is leaving the car
The first o2 should flactuate fast between 0.1v and 1.0v telling the ecu to correct the fuel mixture to a lambda value of 1.0