Checking for a bad o2 sensor

Is there a way besides the common decrease in mpg to check if an oxygen sensor is bad and needs to be replaced? I have replaced my catalytic converters and still receiving a p0420 code. I need to tell if the oxygen sensors are bad or if the code is being thrown because the new converters are not the California emissions grade since my trucks ECM runs on California emissions. Any ideas please!!

 

cali emissions is a lot stricter then normal emissions so they not cali cats wont do the same as the cali cats which means to you need the cali cats which cost a lot more from the dealer

 

Move the sensors side to side. See if the code follows the sensor.

 

^^^yup. This.