How can I tell if my engine block is cracked?

The symptom, my radiator fluid started turning white and gummy and thick, but the car did not overheat and the exhaust was normal, the oil in my engine was low but was normal, so the mechanic said it was the head gasket which i replaced. Now its doing it again slower, and now he said that its the engine block, that probably has a crack and now i need to replace the engine. Is this true? or can it be something else?