The way that O2 sensors should be checked is by monitoring the voltage when the vehicle is all warmed up and the ECM is in closed loop mode. Running the engine for 5 minutes will ensure that this is the case. Connect your high impedance voltmeter to the sensor, leaving it connected to the circuit. The O2 sensor voltage should fluctuate fairly rapidly between .4v - .6v. This indicates that the sensor is responding quickly to changes in the mixture caused by it's operation. A faulty sensor will either have a constant voltage output of around .5v, or the voltage fluctuations will be slow (less than one every two seconds). An output on the high side means the engine is running rich, an output on the low side means it's running lean. You can play with the mixture by pulling a vaccuum line or feeding the engine propane from an unlit propane torch to check out the operation. Incidently, there are several sites on the web that have DIY articles on building a LED dash indicator that will display the output of the O2 sensor constantly - it is a very close indicator of the air/fuel ratio, .5v being a 14.0 - 1 mix. One such site is at: http://www.euronet.nl/users/fo_elmo/afdisplay.htm
Richard Wooldridge