If there's not a tag on it you'll need to raise the rear wheels off the ground, mark reference points on the driveshaft and tire, and spin the wheel/tire while counting the revolutions of the driveshaft. One turn of the tire will yield some odd number of turns on the driveshaft, like 3 3/4 would tell you it's a 3.73 to one. I'd look at doing ten turns of the tire to yield the most accurate numbers - ten turns giving 37 1/3 turns tells you 3.73, 41 and change tells you 4.11's, or almost 31 points to 3.08's.