Engines are happiest when everything stays where it belongs: oil in oil passages, water in water jackets and combustion gases in the cylinders. That's the natural order of things. But a blown head gasket can result is an engine that sometimes breathes oil, lubricates itself with coolant and tries to cool itself with hot exhaust gases. Somewhere in this chaos are the exact symptoms you need to confirm the failure; you just need to know where to look.
What signs a blown head gasket manifest depends a lot on the engine, where the gasket has blown through and how big the blowout is. If the gasket happened to have blown through between a pair of cylinders with no water jacket between them, and the leak was small, you might not notice much apart from stumbling and misfire. If, on the other hand, the gasket blew through from the combustion chamber to an oil passage, coolant passage or both, you'll end up with leakage from any one of these to any of the others. One classic sign is white, coolant-smelling fog coming from the tailpipe. This is actually steam, the result of coolant entering one or more of the cylinders. You may also get blue or gray smoke, indicating oil in the cylinders. Black smoke that reeks of fuel indicates that one or more of the cylinders is misfiring.
This is a classic sign of a blown head gasket, but can be difficult to recognize if you don't know what you're looking for. Unless you allow the engine to sit for days, coolant in the oil won't simply pool on top of the oil in the oilpan. If that were the case, you'd see water on the dipstick above the oil line when you checked it. Most of the time, though, the water will emulsify into your oil like vinegar in a bottle of Italian salad dressing after you shake it. The tiny bubbles will cause the oil to get lighter in color and go very opaque -- this is the dreaded "chocolate milk" of head gasket failure. If your oil is cleaner because you just changed it, it will be the same color, but be very opaque and hazy. Water-tainted oil also tends to run down the dipstick in odd ways, parting, separating and beading off instead of coating the dipstick smoothly. You may also see and smell steam coming from inside the engine when you remove the oil fill cap.
The old mechanic's spot-check for combustion gases in the coolant is to remove the radiator cap -- with the engine cold -- start the engine and smell the gases coming out of the radiator. Sometimes leaks like this won't become apparent until the engine heats up and the metal has expanded, so it may have to idle up to to temperature first. Exhaust gases in the coolant are often immediately visible as fizzy bubbles rising through the coolant with the engine running, and are often recognizable by smell to a trained nose. But if your nose isn't trained, or you don't want to stick your face near churning, boiling water, you can use a "block checker"-type dye tester. These kits use a special dye that turns from blue to yellow or green in the presence of combustion gases. The kit comes with a test cylinder that you fit over the radiator cap opening; if the fluid changes color after exposure to the gases from the radiator, you've got a blown head gasket, cracked head or cracked block.
A compression test is often the most definitive, and can be the only way to diagnose blow-outs between cylinders. Start by removing the fuel pump relay, and running the engine till it dies -- assuming it will start. If not, unplug the fuel injectors, or disconnect the carburetor fuel line and drain the carburetor fuel bowl. Next remove the spark plugs. Check them as you do. If you see one or two that look suspiciously cleaner or newer than the rest, it's likely because they've been steam-cleaned by coolant going into that cylinder. This is a definite sign of a blown gasket. The same is true if one or two plugs are covered with wet oil, or oil-soaked carbon. Once you have the plugs out, screw the compression tester into the plug holes and check the pressure while an assistant cranks the engine over. Record the readings from all cylinders and compare them. You're looking for even psi readings from all the cylinders, plus or minus about 10 percent. If most of your cylinders are reading 180 to 200 psi, and two cylinders are reading 25 psi, you've found your blown gasket.
