If you look in the workshop manual you should see a table of resistance versus temperature for the sensor. So it's easy to check the resistance of the sensor with your multimeter and compare to the table.
I don't know the positioning of the sensor on that model, but in many cases there is a problem with the sensor being screwed into the socket, and there is a void inside, this means the sensor never heats up properly, so telling the ECU the engine is still cold, so you get a richer mixture. The solution in that situation is to add some copper grease or heat conducting compound to the socket before screwing the sensor in.
If it's running rich then the sensor will be telling the ECU that the engine is too cold, complete disconnection would give infinite resistance, but at ambient temperature, (engine cold) the resistance should be in the region of 10k ohms. The resistance drops as it gets warmer down to a few hundred ohms at working temperature.
