Head gaskets have to seal the combustion chambers so very hot gases stay in, and keep coolant and oil out.
You may have a head gasket failure if you see bubbles in the coolant when you remove the radiator cap on top the radiator (not the one on the coolant reservoir) while the engine is running. Bubbles in the radiator could also mean a cracked cylinder head or cracked cylinder wall.
Other symptoms of a bad head gasket include oil in the coolant or coolant in the oil, and of course, white smoke out the tailpipe. This happens when coolant leaks into one or more cylinders, where it boils and turns to steam, which creates the white cloud. You might also find a large puddle of black oil or red fluid under the vehicle. Coolant in the oil reduces the oil's lubricating properties and can accelerate wear & tear on the pistons as they move up and down, leading to a gradual loss of engine power.
If you suspect a defective head gasket, you can perform leakdown test. If you want to do yourself, you will need a leak down tester and a source of compressed air. You can either buy a relatively inexpensive leak down tester ($60-100) or rent one from a tool rental center. You will need a source of compressed air to operate the tester.
There are three basic reasons for cylinder head gasket failures:
As a car owner, you can only control the third reason for gasket failures. Replacing a bad head gasket is the first, and immediate step, but its also important to diagnose and correct the cause of the overheating to prevent a future failure.
The most obvious overheating problems include improper functioning of the radiator, thermostat, water pump, or cooling fan. A dirty cooling system with as little as 1/8 inch (3 mm) of deposits in a radiator may reduce cooling efficiency by 40-percent. Using the incorrect coolant or an improper mixture can have a similar effect. If these all appear okay, then overheating may be caused by abnormal ignition (too early, to late) which can usually corrected by a tune-up back to manufacturer's specifications.
Besides blown head gaskets, overheating can lead to warped cylinder heads, which need to be resurfaced (expensive) or replaced (even more expensive). Compared to the old cast-iron engines, today's all-aluminum or bi-metal (aluminum and iron) engines are more prone to gasket failures, because it's less durable than iron and more prone to heat expansion (but newer lightweight engines have much better power-to-weight performance).