Until a compression leak blows wide open, it's a lot easier for compression/combustion pressure to get into the coolant than the other way around. A gasket or a crack that will leak 120 pounds of compression pressure, or a bubble of hot gas when the cylinder fires won't necessarily let any 15-lb. pressurized coolant go the other way. By the time coolant starts showing up in the cylinders or the tailpipe, things have gotten way out of hand. I've done the cylinder leakdown test, where a combustion chamber is pressurized with the crankshaft locked, looked for bubbles in the radiator, and found some pretty sneaky head gasket leaks that way, but you have to check all the cylinders individually.
Jerry
