One of last month's popular search queries was "blown head gasket symptoms". Here, I shall list a few of them:
One of the first things you may notice if your head gasket is blown is your car will run rough. This will be caused by a loss of compression within the cylinders, which allows the pressure from the combustion of the fuel and air mixture to escape.
Another popular sign is steam from the exhaust with a high rate of coolant (anti-freeze) consumption. This is because on the downward stroke of the piston, the intake stroke, the fault in the head gasket allows for coolant to leak or be sucked into the cylinders, which will then be pushed out the exhaust pipe. Because the exhaust runs at such high temperatures, especially the catalytic converter, the liquid coolant turns to steam instantly, causing a white cloud to be seen from the exhaust pipe.
If either of the above two symptoms occur, check your oil for signs of water. Anyone who has taken a basic science class of any sort knows that oil will emulsify in water, causing the two to separate. When you pull your oil dipstick, you'll notice a heavily broken pattern of oil, and probably a milky substance in between. The milky substance is actually water.
One final thing to check would be your coolant. Look for both a low level/high rate of consumption and signs of oil in the water mixture.
Be advised, all of the signs of a faulty head gasket could also be signs of a cracked cylinder head or cracked block (the main part of the engine). These two conditions are very costly to repair, and typically are used as justification for purchase of a new car.