Look for signs of water in the oil: remove the oil filler cap and look for mayonnaise type gunk in the oil. This is a tell tale sign of a gasket leak or even worse a cracked block.
You could get the compression tested on your engine this will tell you for sure.
Take it to a good independant garage, if they say it's the cylinder head gasket then it probably is. They should be able to give you an accurate quote for the job. If the head has warped it will cost more so avoid driving it with low water contents.
Tim punto
August 2007