"–If you had the codes read, you might be able to tell. "
Actually NO…When you read the codes…if an O2 sensor is bad it’ll give a false reading to the OBD-II system. If a cat is bad…the O2 sensor will set the same error to the OBD-II system. You can’t tell what’s bad by the reading. All you know from the reading is that the O2 sensor is saying somethings wrong. From there you have to do more tests to determine if it’s the O2 sensor or it actually is the catalytic converter.