Ok lets define the types of rear ends that you will encounter from the year cars that you are looking at.
1: You have the open carrier one wheel drive non posi rear end.
2: The limited slip rear end. This one uses clutch packs and will slip some if to much traction difference occurs between the two drive wheels.
3: Locking deferential. Most of these use clutches in conjunction with a locking device that locks both wheels into full lock when a set RPM is achieved.
I have always used this method to check.
With the rear axle jacked up and off the ground (be sure to properly support the weight of the car, preferably on jack stands) the transmission in park or in gear (if it is a standard shift) and the parking brake off, rotate one wheel, if the wheel turns free and the other wheel turns in the opposite direction it is a plane old open carrier one wheel drive rear end. These rear ends can fool you so if the trans is in neutral or the drive shaft is out of the car have a friend grab the other wheel. If the wheel rotates freely with no drag while the friend is holding the opposite wheel, move on to the next car.
If the wheel can't be rotated or is real hard to rotate and the other wheel still tries to turn in the opposite direction but hard to rotate, it could be a limited slip or locking rear end. You can't tell if it is a locking differential or a limited slip until you pull the cover off the rear. If you find one of these check it some more by doing this.
Put the transmission in neutral and then rotate the wheel again. If the other wheel rotates in the same direction have a friend grab the other wheel and try to rotate it in the opposite direction or simply stop it from rotating. If it is hard to stop the other wheel or rotate it in the opposite direction it could be what you are looking for. The clutches on the limited slip and the locking differential can become worn to where it acts like the single wheel drive rear end. If the clutches are in good shape and with the car in neutral you will find that the other wheel rotates in the same direction and you can not hold it. These clutches are set to brake free at about 600 foot pounds of torque. Unless you are Godzilla you wil not be able to stop the other wheel from rotating. So after doing this little test to several cars take the cover off the back of the rear end to the one that you suspect to be what you are looking for and check for sure. I would hate to buy one that I thought was a limited slip or locking rearend only to get it home and find out that the reason it acted like what I was looking for is because the spiders were all gummed up or full of rust.
On GM cars from that era sometimes you can get lucky and find the option sheet still in the glove box. Sometimes it will tell you if it is a limited slip or locking differential.
Just a note: For all around street use I like the non locking limited slip rearends of that era made by Eaton for GM. On the locking differential I have had the locking mechanism come apart from to much torque and trash the rearend. With the limited slip clutch only type, it will give a little before it explodes.
Hope this helps!