Use an ohm meter to check the resistance. I can't remember the value off the top of my head, I'll have to look them up.
Hook the meter's leads to the terminals of the sending unit. Move the float arm, the needle should move with the arm.
Do the same to the bad unit, chances are, the needle won't move when you move the float arm.
EDIT: 8-12 ohms full and 70-73 ohms empty.