Is there any way to test an 02 sensor to see if it's working correct?

I have no CEL lights, and i've been getting the same fuel mileage at every fillup. I just want to check my o2 sensors to see if they're functioning correctly.

Same thing for the cat convertor. I think it's clogged but don't know how to check.

 

Clogged cat use a temp gun will have a higher temp on the output side of cat. O2 sensors volt meter.

 

You can also check for a clogged cat with a vacuum gauge connected to manifold vacuum.
Snap the throttle open to 3K rpm or so and hold it there. Vacuum should momentarily drop, then quickly rise to a number somewhat higher than what you had at idle- and hold steady.
If it does not rise quickly after the initial dip or rises then drops off- the exhaust is clogged.

O2 sensors, I'll watch the fuel trim numbers on a scanner and see if they are reasonable.