Bad O2 sensor or bad cat?

Is there any good way to see if my O2 sensor is what is setting my check engine code or if it is just a bad (or missing) catalytic converter? I bought the car very used in June of 2022 and when I bought it I noticed some exhaust modification had been done and have been driving it since with the P0420 code on and figured the cat was removed or just clogged (I also just checked it and I now have a P0171 and P1121 code which I will deal with separately). I have seen other posts about looking at the fuel trim or voltages from the sensor, but nothing specific. Also I feel like it's the cat because of the bad smell from the exhaust. I also am burning a little bit of oil, and with that being the case I feel like even replacing the cat is not worth it at this point because it will likely get clogged if it is replaced. (I also don't think replacing my PCV valve will do much good at this point either).