Loosing antifreeze

When you have a suspected cracked block or blown head gasket the first thing to do is check the oil for moisture. If your oil looks milky like chocolate milk, its got water in it.
Smell the exhaust, its easy to dectect antifreeze smell coning out of the exhaust. You can also pull the spark plugs and look for one that is much cleaner than the rest. The water kinda steam cleans it and it will be whitish in color.
On a side note..... I just pulled my 340 to do a re-ring and refresh the engine. It was not leaking any coolant at all. The freeze plugs in the back of the block were rusted about half way off. Inside the block there was years of gunk that was sealing it and keeping it from leaking. Honestly, the core plugs are the most likely culprit if they have not been changed before. Get a mirror and a light and start checking them out.