Pull the sender out and connect it up to an ohm meter. Then lift the arm through it's travel very slowly. The resistance should not jump around.
If you don't have an ohm meter, you can eyeball the windings for breaks. Sometimes a magnifying glass is needed to see inside. If there's a break, it's junk.
If you buy a new one, spend the extra $10 from CIP to get the good VDO one. I waited almost a year while they were out of stock, so I gave up and bought the mystery brand part. It was utter garbage. The first problem was that the float had a hole in it, so it immediately sank. I put the float from a German sender in it, then it worked. But only for a handfull of months, then it burned out.