If it is the 6R80, which I think it still is, KingCake has the correct proceedure. There is a dipstick, located under the truck. To check the fill level the fluid must be at operating temp with the engine running, all while manipulating a small dipstick next to a hot exhaust.
Not much fun under a vehicle on jack stands. Easier if you have a lift, or unfortunately, pay someone to do it, which is what I did with my Explorer (has the same transmission behind the V8's in the '06-10 models).