Jack the rear end up off the ground and put it on jackstands then try and turn one wheel in park if it won't turn it probally is a posi center, if the other wheel turns opostite of the direction your turning your wheel it is a non posi center section
IHTHs
If you put in any other gear but park and spin a rear tire and the other tire spins the opposite way, if the other tire spins the same way your turning and the driveshaft matches the tire rotation you have a posi