With a leak down tester. You apply regulated compressed air into a cylinder, and a guage on the tool shows you the percentage of air that's escaping. You could also do this:
1) Take the hose from your compression tester, and remove any valves that are in it.
2) Bring the #1 cylinder to TDC on the compression stroke.
3) Remove the spark plug, and insert the compression tester hose.
4) Attach the hose from an air compressor to the compression tester hose.
5) Slowly open the regulator on the compressor, making sure the engine doesn't rotate, to 75 psi.
6) Remove the radiator cap, and look for air bubbles.
If air bubbles are present, then either the head gasket has failed, or there is a crack that goes into a water jacket.
If no air bubbles are seen, then the problem could be the intake gasket.
The next test would be to pressure test the cooling system with the #1 piston still at TDC on the compression stroke. If there is no noticable coolant dripping into the cylinder (observe with a flashlight), rotate the engine 360 degrees, and repeat. If coolant is now dripping in, the intake gasket is at fault.
Take care,
-Chris
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