My exhaust smokes white smoke

White smoke is a sign of burning coolant. The level in the reservoir will be going down over time too. Keep in mind dripping water and a little steam from the tail pipe is the normal result of a properly working catalytic converter but you won't see that in your mirror.

To test for a head gasket leak, your mechanic will use a glass cylinder partially filled with a special dark blue liquid. He will draw air from the radiator through the cylinder while the engine is running. If combustion gases are present, the liquid will turn bright yellow.