First check the compression by simply spinning the engine over with all the spark plugs removed, and record the compression readings you get on each cylinder. Then go back and add an ounce or so of oil to the cylinder before checking the compression and see if there's a noticeable change. That checks the sealing ability of the piston rings. Neither way will reveal a leaky head gasket, unless it's blown out between cylinders. Locking the crankshaft and putting air pressure into the cylinder, called a "cylinder leakdown test" is a much better way to check for a leaky head gasket. Be sure both valves are closed, put the transmission in gear and set the parking brake and/or chock the wheels, and pressurize the cylinder by brazing an air hose fitting into a spark plug shell. Listen for leakage at the oil filler, intake manifold and exhaust pipe for air leakage that indicates valve problems or leaky piston rings, and look into the radiator for bubbles that show a cracked head or a blown head gasket.
Your pressure test of the cooling system has probably already done a better job of checking the head gasket and head than a compression test or a cylinder leak test can do.
Jerry
