[VetteVet]

Actually you just need to disconnect the sending unit wire at the sender and ground it then with the key on you should see empty.
If you disconnect it and don't ground it you should see full plus about a 1/4 tank.

Measuring ohms , with no wires connected between the tank sender terminal and the tank flange mount you will read the resistance of the sending unit variable resistor which should be between 0 and 90 ohms.