WIN!!!An open diff will almost always spin the pass. side tire.
A Posi will usually spin both tires if it's functional.
A better and safer way to check is to securely jack up rear of car, put trans in neutral, and spin one back tire...
If the opposite rear tires spins the same way, you have Posi-trac, if it spins the reverse way, it's an open diff. Since I'm more of a Ford guy, I can't say with certainty that if the Posi unit is bad, it will act like an open, although Ford's trac-loc does when it wears out. (yeah yeah, now I gotta listen to the anti-Ford shit. ...whatever...)
Spinning the tire(s) to check is kind of....juvenile