Well, there are two studs and the frame is grounded to the dash. The power stud supplies the power to one movement coil directly and suppies power through a second coil to the sender. So, measuring the resistance to the unit frame from each stud separately should give two dufferent values. The one giving lower resistance value is the power lead.
TwoTone, If i remember correctly, The red tab stud goes to the sending unit. Do a little checking with your gauge and use what Truckernix said in a prior post.
