Sometimes an "open" rear end spins both tires. I assume it happens when both tires have equal traction. But don't quote me on that.
As long as you can jack the rear end off the ground and spin one wheel clock wise, and the other one spins counter clockwise, then it's not a posi.

Tom, just take that sucker out in the rain....you will learn all you need to know about driving a short wheel base, high hp car with a limited slip differential.