A properly functioning ox sensor must reach .9-1.0 volts when driven rich and under .1 volts to around 0 volts when driven lean.
Ok, use a quality digital multimeter and set your scale on 1 volt DC, find the signal wire for your oxygen sensor-each model is different so I can't list them but whatever wire you have your A/F guage on, is the correct wire. Single wire sensors are obvious.
Get the car warm and either use a bed of nails on the wire or backprobe the connector. Idling the voltage should toggle rich and lean, the readings are not important at idle. Get the revs up to 2500 and note readings, it should be toggling between rich and lean fairly fast, now let it idle. Grab your propane torch and feed it to the engine, use the air intake or pcv hose-DO NOT cause a vacuum leak or this won't work, feed propane into the engine, the voltage should go up to at least .8 volts and stay there, if no, replace sensor. Now, remove propane and let it stabilize, unplug brake booster and cause a big vacuum leak-it should drop below .2 volts and stay there, if not, its junk.
So to recap, 2500 rpm, the sensor should be toggling-if not and it passes the above tests, it has other issues and the sensor is good. If it doesn't toggle and fails the above tests, its bad.
Note, this is not foolproof but a good way to check for a really bad or blatantly faulty ox sensor-to properly check, a Lab Scope is needed.
12.57@104 :O) 
