Identifying rearend

No, not all 8 3/4 rears are sure grips, in fact in my experience most are not. You can tell the gear ratio easily by jacking the car up and counting how many revolutions of the driveshaft are required for one complete revolution of the rear wheels.
Oldmanrick is correct, it is also common to swap the slightly wider B body 8 3/4 into the A body, and all B body rears have the 5 on 4.5" bolt pattern
From the factory the paint daubs on the outside of the pumpkin told you what ratio, and if it was a sure grip, but they are probably long gone by now.