Well you can locate the serial number and look it up. If you just want to know the gear ratio you can count the teeth on the pinion and the ring gear.
1) Get down on your hands and knees 2) Look under rear of truck 3) See what color rear end is Seriously , refer to info here if truck is stock. https://skydrive.live.com/?cid=9169aad55cfc6d79&id=9169AAD55CFC6D79!354#