Light colored smoke out the exhaust is normal until the engine warms up, especially noticeable on cold mornings dense air. Blueish/gray smoke is a sign an engine is "burning oil", excessive black smoke would be a sign an engine is running to rich/too much fuel is entering the engine. White steam perhaps noticed with a slightly sweet oder from the exhaust, rough engine running, over heating and or coolant loss would be a sign of a blown or mildly blown head gasket.
The most accurate way to test for a blown head gasket on a gasoline engine is to test for the presence of carbon monoxide in the cooling system. Snap-On, Matco (part # CO 2000), and Napa sell a tool to check to carry out this test. Another link to a supplier of this tool is http://www.etoolcart.com/combustion-leak-detector-lis75500.aspx. Carbon Monoxide is only present if there is a blown head gasket or cracked cylinder head. http://www.arrowheadradiator.com/head_gasket_or_combustion_leak_test.htm I think the napa part number 700-1006. http://www.blockchek.com/instructions.htm . Signs of a blown head gasket are coolant loss (frequently have to top up the coolant level of the engine without seeing an obvious leak), experiencing over heating or rough engine running or the presence of white sweet smelling steam from the exhaust. A badly blown head gasket may allow engine coolant and oil to mix.
