Go to Jeep.com and I beleive if you have your vin you can get all the specs that the jeep came with off the lot.
I believe I've read that you can lift the front tires and place jacks underneath behind the lower arms. Make a mark on the driveshaft and have someone mark the top of the tire.
Rotate tire 1 full rotation and count the number of times the driveshaft turns. It will give you the rough estimate. 4 full rotations and slightly more is 4.10 gears. almost 5 reotations of the driveshaft and they are 4.88s