how to tell the rear end ratio...?

Simple. If it is in a car, jack up only one wheel. If not, BLOCK one wheel so it will not turn.

Put the car in neutral, release the parking brake, and see if ONLY ONE wheel turns. If it does, it means you do NOT have limited slip (Sure Grip) of some sort

Now turn the jacked up wheel TWO full turns while carefully counting the driveshaft turns. It will be obvious, as 3.23 will be 3 1/4 turns, 3.55 will be a smitch over 3 1/2 turns, etc.

If the one wheel will NOT turn, and you must have BOTH wheels free/ jacked up, this means either you have a Sure Grip / other type of limited slip, welded spiders, or a "spool," and so now turn one of the wheels ONE full turn, and again, carefully count driveshaft revs.

Another way, if you plan on tearing down the axle for cleaning/ inspection anyhow, is to count the gear teeth on both the ring and pinion. Divide the larger no. by the smaller no. and you will have the ratio.