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Whats the best way to determine the source of coolant consumption? How should one differentiate between say a leaking radiator, leaking waterpump, leaking hoses, a blown head gasket, or crack in the block/head(s)?

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First thing is to initially determine if you're burning it or leaking it. Assuming your car isn't spewing a plume of white smoke when you're driving - you will have to do some additional diagnosis to determine the method of consumption.

First check the oil and coolant - if either contain a milky substance you have a cracked something or a blown HG. Possibly a failed oil cooler (if it's of the water cooled variety) Only a leakdown test can determine anything further beyond a teardown. I have heard of some cars having poor intake manifold sealing which can cause coolant to be sucked into the intake tract and burned without the telltale white smoke.

If the fluids looks okay, have the system pressure tested. Basically it's like hooking up a bicycle pump to your radiator to pressurize the system while cold. If the leak isn't obvious you can use a UV dye to pinpoint weepage or very minute leaks.

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+1 for pressure test. In theory, a pressure test can also be used with the engine running to see if there is a blown head gasket. The high combustion pressure should cause an above normal reading on the pressure gauge. I've never gotten to test this in the field though, as usually when the head gasket is that far gone, it's already smoking like crazy. – S_Niles Mar 9 '11 at 23:23
Don't forget to check your A/C drain when doing the pressure test. Usually if the heater core is leaking, you can smell it in the car, but it's an often overlooked source of leaks. – S_Niles Mar 9 '11 at 23:29
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One way would be to have the vehicle running, and look for where the leak would be coming from. You may want to top off the coolant first for best results. However, if its a slower leak, this technique may not work as well. But, you should still be able to get some indication of where the problem is occurring.

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