Sender out of tank? one end to gauge wire other to ground and move the arm through its range. it should read a steady increase or decrease in resistance with no dead spots. can't remember the exact value but its on here somewhere.
If you get no reading that means it's an open circuit which is a common problem with senders that don't work. If working properly it should show a low resistance between the output post and ground and it should vary as you move the float up or down.
what I like to do is remove it from the car, pop the trunk and pull that sending unit wire up through the grommet and connect it to the sending unit. Then take a jumper cable and connect it to a good ground.
With the key on/engine off I will move the sending unit arm up and down while looking through the rear window at the gas gauge to see how it reacts.