Yes, car in neutral and blocked, both rear wheels lifted, put a mark on one tire and a mark on the driveshaft. Turn the marked tire forward one rotation, and count the driveshaft rotations. A little over 3 times is 3.08, about 3-1/4 turns of the driveshaft would be 3.23, about 2-3/4 turns is a 2.73, and so on. My 1980 Pontiac service manual shows the ratios offered that year are 2.41, 2.56, 3.08, 3.23, and 3.42.
Here's a pic of where the code is on the housing. That's the front passenger side axle tube you see there. It's stamped into the metal of the tube. You may be able to see it if you clean it off with a wire brush and little sandpaper. A flashlight and a magnifying glass might help read it. It'll be a pair of letters that's the code.