Cletis described the test for a sender but you're asking about bench-testing the guage itself, right? Best way to check the guage is to hook the hot side of the guage to the pos. side of a 6V battery and the other side of the guage to a known good sender. Then connect the guage case and the sender ground post to the neg side of the battery. Move the arm up to full, then down to empty, the guage should follow.