O2 sensor test

Is there a full proof way to test the o2 sensor?

Worth the money to buy new.  You know how strict smog can be in Cali.............unless you know someone.

Check FSM, but easiest way for me is, does it change the way it operates when you unplug the o2 sensor? the ECU relays heavily on that o2 sensor, i think disconnected it would run extremely rich if I recall.