Jack up one rear wheel so you can spin it...chock the front tires first *G*...put the trans in neutral. Try to spin the tire by hand. If it spins, go to step one below. If it doesn't, jack up both wheels and go to step two.
First...Mark tire with chalk or other marker on inner sidewall; crawl under car and mark differential yoke and pinion housing where they meet.
Step one...rotate tire one revolution, noting revolutions of differential yoke. Multiply yoke revolutions by 2. This is your gear ratio.
Step two...same as step one except that the actual yoke revolutions are your gear ratio...no multiplying..
If you can spin the tire when only one is in the air, you have an open differential. If you can't, you have some sort of a limited-slip differential and it should require 40-60 lb-ft at the axle CL to overcome the differential when only one tire is in the air...
Remember, make sure the trans is in neutral and the parking brake is not set...