The best way to tell if you don't have the sticker. or the badge on the diff cover is missing:
1. put vehicle in gear ("drive" if auto)
2. Jack up the rear of the vehicle (make sure that both rear tires are off the ground and please chock the front wheels and use jackstands!)
3. Mark the top sidewall of one tire
4. Mark a spot on the rear driveshaft
5. Have someone turn the tire one full rotation
6. Count how many times the driveshaft truned.
Example: If in one full rotation of the tire, the driveshaft turned 4 times, that means you have 4.10 gears; 3 and 1/2 times means 3.55 gears, etc..
Hope this helps