How do you tell posi rear end?

You have to jack up the rear wheels and spin one of them and look to see if the opposite rear wheel spins and if so it would spin in the same direction. A open diff. will have the opposite tire spin in the other direction and a limited slip will some times leave the other wheel standing still and slowly start turning in the same direction after a certain amount of revolutions are made by the wheel you are spining by had to check as these work off of clutch plates and need friction to engauge them.