1999 E300 Coolant loss question

[ROLLGUY]

My friend has a 1999 E300 that seems to be loosing coolant. He had the system checked for leaks, and the rad cap was suspect. A new $35 cap from Pelican, and it still has coolant loss. I suspect a bad head gasket. Anyone know how to make sure of a bad head gasket? I have heard of a test for combustion gasses in the cooling system, but would that be a definitive test? The engine is not overheating at all, nor is there any external evidence of a coolant leak. Also, he says the engine "shudders" when it is turned off. I think he means that the engine shakes a little upon shutdown. I don't know if this is a related problem, but they both started at the same time. Thanks in advance, Rich