fuel tank sending

Ok had this problem when first started to restore the Dart.My fuel Gague reads way empty now the sending unit is only 3 years young.could someone refresh me on how to test to find the problem.If I remember right if I pull the wire off the unit and ground it the gauge should go to full meaning the gauge is good and the sending unit is bad right? I do have a ground wire back there.So if I ground that wire and nothing happens were do I look next thankssssssssss fabo.