Easy rough guess way: Jack up the ass end, put a mark on the drive shaft and a mark on your tire. Spin the tire until the drive shaft makes a complate revolution back to where it started. Count the amount of times the tire rotated and that'll give you a pretty close estimate.
Let's face it, there's only so many ratios it could be, assuming it's the stock axle.
Whoops, you're right. CC to the rescue once again, thanks.ya got it backward sport. 1 revolution of the tires and count how many times the DS rotates.
DS 3.5 turns= 3:55, 3 3/4 turns = 3:73, just over 4 turns 4:10 etc.