Originally Posted by GSGALLANT

Use the dipstick to verify that you have the correct level in the pan. The proper method of checking transmission fluid level is with truck running and warmed up (there are cold level hi/lo lines on the dipstick also, but the most accurate measurement will be obtained using the hot hi/lo lines.

So, with the truck running, slowly cycle through all the gears from "P" to "L" and back up to "P" with your foot on the brakes. Then, leave the truck running, and pull the dipstick out to check the level. If the the truck is already warmed up, use the "hot" lines on the dipstick, otherwise, use the "cold" lines on the dipstick. The dipstick should be located on the passenger side of the engine, near the rear of the engine compartment. Like others mentioned, it should have an orange handle.

If you've just come in from a long drive a high speeds on the highway, let the truck sit (not running) for approx 20 minutes, then come out and start it, cycle through all the gears, then check the level. If you don't let it sit for a bit, your indicated level will be too high, due to your tranny fluid temperature being hotter than "normal operating temperature."

ATF fluid is added (if level is too low) through the dipstick tube, and is drained (if level is too high) using the drain plug.

Good luck.