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Is there a definitive way to test whether head gaskets in most engines are blown, without actually reaching for the gaskets themselves (at which point you might as well go ahead and replace them) assuming that the engine is overheating and that all the components of the cooling system are checked positive and are not leaking?

I have heard of block testing as well as using coolant that has dye in it, which would leave trace on the spark plugs. Are these good ways to check and are there any other?

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There are a lot of Quick methods to check the Head Gasket.

  • Coolant leaking externally from bellow the exhaust manifold
  • White smoke from the exhaust pipe
  • Overheating engine
  • Bubbles in the radiator or coolant overflow tank
  • White milky oil
  • Significant loss of coolant with no visible leaks

These are usually important points to notice while buying a Used Car.

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To add to Anarach's answer...

There are 4 things around a head gasket - combustion chambers (cylinders), oilways, coolant-ways and the outside air. They can fail between any two (or more) of these, and each has different symptoms.

oil-coolant failures will result in one or both being contaminated - look for a mayonnaise-like substance in the oil (though small amounts of this can also form from condensation, particularly if the engine has been sitting for a while), or oil in the coolant.

oil-air or coolant-air failures should be obvious, with deposits of either on the outside of the block.

oil-cylinder or coolant-cylinder failures are probably most common. They will result in a smoky exhaust (as the contaminant is burnt), and pressurisation of the oil or coolant systems (leading to loss of coolant and the overheating that is commonly the first obvious symptom). They can also be detected by a compression test, which will check the amount of compression in each cylinder - a failed one will be much lower as the air will be escaping.

cylinder-air failures are rare (though you can also get cylinder to cylinder failures, particularly in over-bored engines). Again, a compression test is a good indicator...

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On top of the other 2 answers...

To test a head gasket blown between the cylinder and another port, do a leak down test. This pumps air into the cylinder and measures how much is lost. You can listen and look for the source of where the air is exiting.

To test the coolant system, do a coolant system pressure test. This is similar to the leak down test. Pressurize the coolant system, and see how much pressure you loose. If you loose pressure, it means coolant is leaking out somewhere.

You can purchase both of these test kits at Harbor Freight for about $50 each.

Always give your engine a good cleaning before hand so it is easy to spot the source of leaks.

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Cylinder Based Tests

Pros

Useful in isolating the source of a gasket leak at the cylinder level.

Cons

Can be inconclusive due to other sources that can also yield compromised cylinder seals via piston rings and/or valves.

Since cylinders are tested on an individual basis one may need to test all cylinders before the failed one is discovered.

Examples

  • Compression Test
  • Leakdown Test
  • Visual Inspection: coolant leaking into a cylinder will power wash the carbon off the cylinder head which can be seen and evaluated with an endoscope through a spark plug port as demonstrated here. The crankshaft may need to be rotated to line up the cylinder head to the proper focal length of the camera to put the surfaces of interest into focus.

Block Tests

Pros

Useful in testing the entire block at once, i.e. all cylinders.

Cons

Less useful in that the compromised cylinder(s) cannot be identified

Examples

  • Chemical Test: uses a plug and siphon device to seal off the radiator cap port so that any leaking combustion gases into the cooling system can be suctioned into the device and detected by a chemical reaction which changes the color of a special solution. A bulb aspirator is typically used to draw in air from the coolant system to mix with the solution and exploits the fact that the concentration of CO2 in such air from a leaking head gasket is going to be significantly higher than the 0.04% found in the atmosphere. A control test with exhaled breath is used to determine the solution is still good as demonstrated in this video, since a reaction/color change is expected under the concentration of CO2 in exhaled breath which is 4% or 100X more concentrated than atmospheric CO2. The concentration of CO2 out a car's exhaust can be as much as 16%.
  • Gas Analyzer: used to measure hydrocarbons, i.e. uncombusted fuel. Demonstration by ScannerDanner using the fga-4000-xds
  • Smell: I've seen a technician smell a radiator and claim to smell exhaust or something he felt suggested a bad head gasket.
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  • Why did you community-wiki this? – Zaid Jan 26 '17 at 7:03
  • I'm curious on this question too, and I think a lot of the answers above are good but focus on symptoms and evidence of head gasket leaks, but I'd like to try to open a wiki for positive active tests initiated by the technician to verify a head gasket leak rather than enumerating affiliated symptoms. Any commentary on specificity and sensitivity constraints and what not would be ideal since the ScannerDanner vid above finds a head gasket leak where the chemical test was not sensitive enough to register it. – jxramos Jan 26 '17 at 7:03
  • I made a wiki since I think there's a deeper commentary on the effectiveness of each test and what exactly it demonstrates and does not demonstrate than I can offer, eg leakdown can source which pair of systems have a broken barrier between them while a compression test cannot distinguish such things, etc. – jxramos Jan 26 '17 at 7:06
  • I think another issue is that often some tests show a failure in one of a few potential systems, eg bad valve seal, or rings, or head gasket and you need to refine the test and build a body of evidence to conclusively narrow things down. – jxramos Jan 26 '17 at 7:37
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    There are technical publications that will give you the details of what to look for and the possible conclusions for each of the tests you mention above. Such as (one example of many possible) Engine Testing: Theory and Practice by Gerard Meurant. – Solar Mike Jan 26 '17 at 12:51
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Dude... All you have to do is disconnect the exhaust at the 'y' or after the headers on each one... Make sure the radiator is full and crank it over. You can disconnect the coils if you want... And whichever pipe leaks water is the associated head that's cracked. We all know its quite rare for there to be an air leak. The engine definitely would have shut off by overheating before that. Your information is extremely valuable- but its overkill. I mean we arent going to change out the gasket around a specific cylinder only- so it matters not which cylinder has the problem- or why. Just the head. If you have to cut the pipes-cut them- just be sure you leave enough for reconnection piece and a clamp.

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