Again... If you have an O2 sensor installed and if you DISCONNECT it, it can be used to measure the fuel-to air ratio with very little oscilation. Caveat, if the F/A ratio is really out you'll get some major voltage fluctuations until you are in the 0.45-0.50 milivolt (if memory serves me) range.
The O2 sensor is simply an oxygen reactive metal that produces its own voltage which varies with the percent of oxygen in the exhaust gasses (and that's why you can use it to adjust your F/A ratio by measuring that voltage). The car must be warmed up so you aren't reading the rich warm-up period and the engine should be "revved" between adjustments. This is about as accurate as you'll getting without the right equipment. Put one end of you voltmeter into the single wire exiting the sensor (any additional wires are for the heating element and will read abou 12 volts, usually) and put the other probe to ground; when you're reading less than 1 volt you're probably on the right one.
The sensors do wear out, some sooner than others. Some just start reacting too slowly. So, start with a new sensor>