You'll find the ratio code and the assembly date stamped on the bottom flange of the diff housing, between the spring retainer plate and the camber strut rod bracket. Will require some heavy cleaning to see the stamped characters.
Or you can jack up the rear of the car, put it in neutral and have someone spin the tire one full revolution and you count how many turns the driveshaft makes. My wheel once fully around turns my drive shaft 3.08 turns! Not that I could measure the .08, but I know it wasn't 3.5 or a little past 4... :cool