Or jack the rear up and put it in neutral. As you have someone turn a rear tire slowly one full revolution, get underneath and count how many times the driveshaft turns.
If the driveshaft turns about 3.5 times for that one revolution of the tire, you have 3.55 gears. If it the driveshaft turns just over 3 times, you have 3.07 gears. Etcetera, etcetera.