When the float is at the bottom, the slide wire on the sender has about 72 ohms, When the float is at the top the slide wire has about 10 ohms. They are easily checked on the bench with a good ohm meter.
If you ground the wire that connects to the sending unit and the gauge is working correctly. The gauge will go to full and beyond. If you do this, just do it for a moment as you can damage the gauge if you leave the wire grounded for more then a few seconds.
This is just a gauge test and not a sending unit test.