Sender units are always inside the fuel tank. On top of the tank can be seen the 'base' of the sender unit with two wires attached [+ and -] - usually the unit can't be removed without removing the tank. If you can remove the two wires and connect together completing a circuit to the fuel gauge, then the gauge can be tested. With wires connected and ignition 'on', if gauge is working it will show 'full fuel'.If it fails to move the sender unit in the tank is probably stuck or faulty. However check the circuit for conductivity - could be a broken wire in the circuit but this is unlikely. Obviously do not put any volts into the fuel tank .... BOOM !!!!!!!