You will find the ratio numbers just inside the edge (flat area between the bolt threaded ends)of the ring gear(teeth side). You will need to pull the rear cover to see the numbers. OR pop the axle shafts out a few inches,drop the driveline and pull the diff assembly out to inspect it.
The other option is to jack it up and count the turns of the driveline to the turns of the wheel.If it is a no-slip diff, you will need to jack up both wheels.
Most common ratio will be 4:11