Yep. If it has a posi unit, it's alot easier, but you can still do it with an open diff. Jack up the rear of the car (so the tires are off of the ground) and put the trans in neutral. Place a mark or tape on the inside of both tires, pointing straight to the ground. Also, place a mark on the drive shaft. Now, turn the drive shaft until the tires make 1 complete rotation, and count how many time you rotated the drive shaft. If it's an open diff rear, you may need to remove the tires and brake drums so the brakes won't drag. What you're shooting for is both axles to rotate the exact same amount, so you can get an accurate count.
ex. Tires rotate 1 time and you turned the driveshaft 3.5 times. This means that you have a 3.50-3.55 rear gear.
On the tag question: if no one has removed it, or changed the gears, there was usually some kind of tag on the bolts of the rear cover that identified the rear gear ratio, but if you're like me, luck would have it that someone has removed it.
