8 3/4 posi ?

a friend of mine just asked me a question about a 70 dart he's thinking of buying.
The owner is not sure if it has a sure grip in it ,the car is not running at this time.
If you jack it up and spin the rear wheel and they spin in opposit direction does that mean it has posi, don't want to give bad info. He thinks I should know because I have a 70 duster but I don't.