Some times there is a tag on the rear end cover with the gears stamped on it. But if the gears have been changed the tag could be wroung now.
The only way I know of with out yanking the rear apart and counting teeth is to.
Jack the rear of the car up. Put a mark on the drive shaft. Turn the rear tier one full turn counting the turns of the drive shaft. if the drive shaft turns three and a half times you have 3.55 gears if it turns all most 4 times 3.91 gears and so on.