This is probably the easiest way to measure your rearend ratio, as it doesn't require removing the differential cover. Jack up the rear end of the car (so that both rear tires are off the ground) and make a mark (or pick a spot) on both the driveshaft and a rear tire. With the transmission in neutral, spin the tire by hand through one rotation, and count the number of times the driveshaft turns. The number of driveshaft rotations is your rearend ratio. (For example, if the driveshaft turns about three and a half times, the ratio is probably 3.55. If its almost three and a quarter, the ratio is probably 3.21. Almost four times, 3.91s, etc.
Note: The procedure above applies if you have a Sure Grip (limited slip) differential or a locker. If you have an open differential, the procedure is the same, but you will need to spin the tire through two rotations instead of just one.