How do I tell if a 7.5 has posi?

[Sinister G]

I need a posi in my 86 Cutlass, it's current stock rear end is crap and about to blow.
I want to just go to a junkyard or maybe Craigslist and get a 7.5 posi with the 3.73 out of a Monte SS.
But how do I make sure it's a posi and in decent shape? I don't wanna get ripped off and I know how people will make stuff up just to sell a part.

And what should one of these rear ends usually cost in good working condition?

 

[CHRIS.O]

2 easy ways.
1. Get the rear of the car off the ground, both tires need to be in the air. If you can get it in neutral spin one tire, the other tire will turn the same direction if it is a posi. If it turns the opposite way it is an open diff.
2.Take the cover off, if you only see gears then its open. A stock posi unit will have springs instead of just spider gears.

 

[RITTER]

2 easy ways.
1. Get the rear of the car off the ground, both tires need to be in the air. If you can get it in neutral spin one tire, the other tire will turn the same direction if it is a posi. If it turns the opposite way it is an open diff.
2.Take the cover off, if you only see gears then its open. A stock posi unit will have springs instead of just spider gears.

X2

(sorry everybody, inside joke)