I assume you are talking about the electrical senders with needle-type gauge that mounts into the top of the oem float unit as opposed to the totally mechanical needle gauge. On our boat, the water tank uses one of these while all other gauges are totally mechanical with no sensor.
While the sensor is mounted on the gauge, unless you can cause the float to move up/down while you are testing them, you won't be able to determine much with the MM except for the resistance reading in the sensor at the float's current point. Obviously if you got an open circuit reading, it would tell you that the sensor is not working at all. HOWEVER...
The sensor is magnetic; You can remove the two screws that hold it to the top of the gauge. You can then take a small magnet, hold it against the bottom of the sensor and rotate it. The needle should rotate and the resistance read by the MM should change as you rotate the needle (using the magnet) from full to empty. Offhand I don't know what the resistance reading should be from full to empty but if you have a constant change as you rotate the needle, the sensor is OK. CAUTION: when you put the gauge back on the float unit, be sure you have the gauge's needle in EXACTLY the same position it was before you removed it. Otherwise, the magnetic "connection" will not be correct and the gauge will read improperly until the tank is emptied.
Checking in this manner will only tell you if the sender is working; it will not tell you if the entire assembly - float and gauge - are working properly.
On a related item, I just purchased 2 new float gauges to replace the old water tank ones. They are not cheap - around $120 apiece and I recommend you not bother. The float sticks on one of them and you have to bang on it to get it to work. I've fooled with it a lot but have not been able to get it to work. Since it is under the master stateroom bed (has the electric sender), it is not conveniant to bang on it whenever I want to check the reading. For that reason, I am not fooling with them anymore and plan to put the Snake River system in to monitor the tanks.
