Should be wire running to the tank with the fuel sending unit in it. Pull sending unit, ground it and have a helper watch the gauge for movement as you move the float arm up and down. If you don't get any movement put positive lead from ohm meter on wire from the dash gauge and the ground lead from ohm meter to the body of the sending unit. Move the float arm up and down and the meter should show movement through the whole range of the float arm. Don't remember the ohm range is but you should see movement on the meter anyway. If you have full movement on the meter the sending unit is good and the problem is with the gauge. There are different ohm ranges depending on the manufacturer. ie if you have a gauge with a 60 to 100 ohm range the sending unit has to be a 60 to 100 ohm range. If one or the other has been changed over the years and the ohm range is not the same on them you will not get a good reading.