Are you familiar with a combustion leak test? It's a procedure designed to detect combustion gases in the cooling system, indicative of head gasket failure *or* a cracked engine block. Head gaskets typically fail before the block itself cracks, and the latter is caused by the engine severely overheating. You can buy combustion leak testers at Harbor Freight or any auto parts store.
Depending on the severity of the leak, cranking the car with the radiator cap off is a quick way to determine if engine compression is being pushed into the water jacket. If someone cranks the engine and fluid gets pushed out of the radiator, you likely have a head gasket leak at a minimum. Just make sure the radiator is filled for that test. Water will do.
By the way, which engine are we talking about? Lower intake manifold failures are common among W-bodies too.