Jack the car up at the back, take it out of gear/emergency brake off, then rotate the wheels and count the number of revolutions on the driveshaft to turn the wheel once. Use chalk or paint stick to mark the tires and driveshaft if you have having trouble. That will give you your ratio.
Example: Wheel turns once, Driveshaft turns 3.73 times, you have 3.73:1 gears. Stock would be 3.27
-Matthew