Is there a way of telling what the ratio of a drive is if the decal is not on the drive, without taking the drive apart? Possibly like a car, turning the drive shaft and counting tire rotations?

turn input shaft - count prop turns.

That will work, but you are trying to count partial rotations on the prop shaft. Better way (something of a workout however) is to turn the prop shaft 10 turns, count the input shaft turns & divide:

15 input / 10 prop = 1.5

If you can't count the prop shaft vs input shaft rotations because the drive is installed, get the serial number off the upper section and go to Merc's Parts Express. There you enter the serial number and all the information will be available.....as long as nobody altered the ratio since it was built that is.