The easiest way to tell, is to jack up the rear tires. With the trans in neutral and the brake off (preferably on level ground), rotate one tire. If the tires rotate in the same direction, it's a limited slip(posi). If they rotate in the oposite direction, it's a standard differential.

The rear end "tag" is actually stamped on the passenger side, towards the front, of the axle housing. You may have to get some spray cleaner and maybe a wire brush to actually see it. It gives the gear ratio, build date, assy plant, and work shift.