I don't think you can just ground the sensor to tell if it's good. The sensor can tell the difference between water and diesel since they each have their own resistance (not the same). I'm assuming since it is a one-wire sensor, it's "measuring the resistance between the sensor tip, through the liquid, and ground (the bowl). I guess you could always see what happens if you put the sensor tip in a cup of water that also had a grounded wire in it. Cheers!