If you don't have the tag on the diff, then you need to count the number of turns the driveshaft makes to one revolution of a tire. I believe you then double your answer if you have an open (non posi) diff. ie, if your drive shaft makes 1 1/2 turns to one revolution of a wheel, then its a 3.00 to 1 ratio gearset.
It does't affect the ratio but if you jack one side up with an open end and one tire is planted, the free wheel will spin twice. (like you're making a turn) 1x2
If you have LSD, you jack both sides up and both wheels will turn one rev. (like you're going straight) 2x1
If your pushing the car on the ground and both wheels are turning. 2x1
2x1=1x2
You are correct, count wheel revolutions. But sometimes people get confused and count only one wheel while both are turning.
If you don't have the tag on the diff, then you need to count the number of turns the driveshaft makes to one revolution of a tire. I believe you then double your answer if you have an open (non posi) diff. ie, if your drive shaft makes 1 1/2 turns to one revolution of a wheel, then its a 3.00 to 1 ratio gearset.