Just found this on the net. A bit like a breathaliser for radiators. Don't know where you can get it done though.
"The most effective and accurate diagnostic test to determine if the cylinder head gasket is sealing combustion gasses is to check for the presence of combustion gasses in the engine coolant. This must be done with the engine warm and the radiator cap removed. This can be a bit tricky so be careful when warming the engine with the radiator cap removed or removing the cap when the engine is warm. With the radiator cap off and the engine warm, place a funnel where the radiator cap would normally be. Start the engine and let it run. Watch the coolant as it circulates. It is normal to see the presence of some bubbles in the cooling system, but the presence of lots of bubbles in the cooling system may be a sign of cylinder head gasket failure. Be very careful when running the engine with the radiator cap removed. Hot coolant may be expelled from the radiator unexpectedly.
With the engine running, a tool (pictured) is used to draw fumes from the cooling system through a chemical that checks for the presence of carbon monoxide (CO) in the cooling system. Carbon monoxide will only be present if the head gasket has failed or the cylinder head itself is cracked. The chemical starts off blue in color, but turns green or yellow in the presence of carbon monoxide. This test is known as a “block check."
Another method is to use the gas analyzer that the shop uses to check vehicle emissions. When this machine is used, the presence of hydrocarbon (HC) gasses escaping from the coolant will be measured and if the HC reading is over 50 PPM, then the headgasket is leaking. Always make sure the reading is noted on the repair order whenever this method is used."
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