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.

Answered by caradiodoc (expert)
17,285 answers provided