The gear ratio your truck came with is printed on a decal under the hood (Gen I and Gen II) or in the glove box (Gen III). Also, there is sometimes a metal tag attached to one of the bolts on the differential cover with the ratio stamped into it.
However, even if the decal and tag are still in place that only tells you what your truck was supposed to be built with. There is always a chance that a mistake was made, or a previous owner had the gears changed. If you want to be absolutely certain, you need to measure the ratio yourself. Fortunately, this is fairly easy, and there are two ways to do it.
If the cover is off the differential, count the number of teeth on the ring gear and the pinion gear. Divide the number of ring gear teeth by the number of pinion gear teeth. The resulting number is your gear ratio. (For example, a ring gear with 39 teeth and a pinion gear with 11 teeth: 39 divided by 11 is 3.5454...; these are 3.55 gears)
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 truck (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. If you do not know what type of differential you have, see elsewhere in the DML FAQ to learn how to determine this.
If you do not have a jack, you can also do this test on a level area by rolling the car enough to spin the tire once. Obviously, be extra careful if you do it this way (get a couple of buddies to help you) - jacking up the rear is safer.