The old standard is to raise the drive wheel off of the ground, note/mark the orientation of of a fixed spot on the driveshaft (say a u-joint cap facing directly at you), note/mark the orientation of the bottom of the wheel. slowly rotate the tire one revolution while counting the revolutions of the driveshaft. This should get you close. If the driveshaft rotates say just over 4 complete revolutions to one revolution of the wheel then a close guess is a 4.11 to 1 rear gear ratio.

If yu need it more accurate than that you will need to pull the cover and start counting.

Sometimes there is a tag on one of the bolts on the cover that will tell you but you cant trust that. Someone could have changed the ratio and left the old tag on it.