If open wheeler, make sure the car is out of gear, chock the front wheels, jack the drive wheel up (usually the R/H side) and spin it two full turns. Count the rotations of the tail shaft/Drive shaft/Prop shaft and you will have your ratio
If it is a LSD, make sure the car is out of gear, chock the front wheels, raise both rear wheels and turn the tail shaft until the wheels make one full rotation, the amount of times you need to turn the tail shaft is your ratio
For both diffs if the tail shaft turns 3.5 times you have a 3.5:1 diff ratio
The reason for turning the open wheeler twice is because you are simulating cornering and the inside wheel would spin at twice the speed.
Hope this helps