The simplest that I can think of would be to find the wire coming up from the O2 sensor (mine run up on the right side of the firewall, under the battery tray, and into the cabin), and at one of the connections, probe it with a multimeter.
If you can, put a wire in to remote it into the car, or tap it inside the cabin. That way you can sit passenger seat and monitor it during a run while someone else drives.

At cruise, with the engine warmed up, it should fluctuate around .5 volts, which indicates an air/fuel ratio of 14.7:1. If you are reading steadily below or above that, your mixture is off, or possibly the O2 sensor is shot.

If you are getting no reading at all, you may have a bad wire to the sensor, a bad connection to the main sensor wire, a bad sensor, or the Lambda control module that the sensor wire goes to could be shorting it internally. In that case, try unplugging the wire from the main harness if yours will allow it, and probe it with only your meter hooked up. If that shows voltage, but not while hooked to the car, it's likely a bad control module.

All told, if you can find a diagram of that connector on the fender, you may find that one of those pins is the reference signal from the O2 sensor, and would avoid the whole hassle of trying to find the wire under the dash. It adds extra distance to the wiring though, so more possible damaged wire over the years.

I hope this helps more than it confuses!

__________________
-Josh
Testing the cheap Mercedes axiom, one bolt at a time...