The needle will climb too high, basically the whole cooling system will not stabilize and also you may find water in the oil and oil in the water.
THINGS TO LOOK FOR
If you have a suspect blown head gasket and you are driving, stop the engine for 1/2 hour, then very carefully lift the bonnet and using a rag or a set of gloves, gently remove the radiator cap. Remember the radiator cap has 2 clicks. Turn the cap with your hand. Do not put your head right over the top of the radiator when you are opening it, move your head right away, just in case it decides to explode.
It could cost between $550.00-$2,000.00 depending on the make and model. If you have a suspect head gasket, have the vehicle towed.
This article is found in the Virtual mechanic CD Rom
You can download it for the price of a latte, but you will learn not to buy a lemon
By Darren Gow-Brown, Melbourne Australia ©2009