You can monitor the O2 sensor by connecting the (+) lead of the voltmeter to the signal wire by backprobing the wire and the (-) lead to ground. There might be another wire if it is a heated O2 sensor which would read at 5 volts.
After letting the engine warm up a couple of minutes readings should be between 0.1 and 0.9 volts.
A reading less than 0.45 volts is a lean condition.
A reading greater than 0.45 volts is a rich condition.
This will let you see if the engine is running rich, lean or just right (going from rich to lean, to rich quickly).
Hope this helps.
Adam
