The o2 sensor is actively used when the system is in closed loop operation. Then the ECU is correcting the mechanically adjusted fuel (hex tool adjusted through air filter box) injected. This regulation is carried out by a small pressure regulatior (EHA). That means that if your basic setting is way off, the possibility of correcting maybe insuffient even though your sensor is acting correctly.
As indicated already you should get hold of a DVM with duty-cycle setting and connect to the X11 plug (on the left fender) connecting to pin 2 (ground) and 3 and see if you get a reading that fluctuates around 50%. If it is stuck at either high or low reading, then try to bring it back by adjusting the mixture screw.
High reading -> set leaner by anti clockwise adjustment and vice versa for low reading. If you get a fixed 50 % reading (no fluctuation) when engine is warmed up, you know you have faulty o2 sensor.
Alan! I don't agree! You will not by default get a rich mixture, you will have what you have mechanically adjusted it to, lean, rich or correct.