When my hg failed in my 02 kj I had a milkshake in the crankcase. I have used a pressure tester to apply pressure to the cooling system and found an "external" hg leak on a Subaru. If you're leaking alot from combustion chamber into coolant you'll know by the boiling coolant. If you're getting exhaust in the coolant from a smaller leak, there are kits to test for exhaust gases in a coolant sample.