Why is my truck blowing white smoke?

White smoke is normally a sign of burning coolant due to a leaking head gasket. If that's what's happening, the level in the coolant reservoir will be going down.

Have it chemically tested-

How can you tell what? If the level is going down in the reservoir or if the head gasket is leaking?

The level in the reservoir will go up and down when the engine heats up and cools down, but over time it will run empty if a head gasket is leaking.

Most of the time combustion gases will also sneak out through the leaking gasket and blow into the cooling system. From there the gases will make their way to the reservoir. If you see bubbles in the reservoir while the engine is running, that's pretty good proof a head gasket is leaking, but if you don't see bubbles, that's not conclusive. Those bubbles will make it look like the coolant is boiling, but you know that's not the case if you see them when the engine is still cold.

When you don't have bubbles, the test rasmataz is referring to, (I call it the "sniffer" test), involves drawing air from the radiator through a glass cylinder with two chambers partially filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.

You also have to consider how much smoke you're seeing. One of the byproducts of a catalytic converter that's working properly is water vapor. Look at what's coming out of other vehicles' tail pipes when they're idling at a red light. If yours is similar, you may not even have a problem. If yours is making enough smoke to turn peoples' heads, you have a problem.