If you took the sending unit out of the tank already then you can bench test it. Inside the tube is a float. On the bottom of the float are two small magnets. The float drifts over a circuit board. One wire on the ohm meter to each terminal on the sending unit. Then, tilt the sending unit to cause the float to slide down. The ohm meter reading should change as it passes each stage. If one or more of the magnets has fallen off the float then the numbers won't change as the float drifts across the circuit board.
In that case just disassemble the sending unit from the top. Fish out the magnet(s) and epoxy them back into place in the grooves on the float. Then, while it is out connect the ohm meter to the terminals and move the float over the circuit board. You should see the numbers change.
Put it all back together. Then test it again with the float inside the tube. It should still work as you tilt the tube upward and the float slides over the circuit board.
Someplace on this site is a thread I put together with pictures that shows how to do everything I just talked about. Maybe run a search with the key words sending and unit.