4.6Northstar coolant missing

How would I go about diagnosing which head gasket is leaking on a 97 deville concours and where could I find more info on how to repair the prob. It overheats. I assume its a head gasket bc the overflow tank bubbles over when the cap is off and I try to start it. It also eats the coolant, no white smoke no visable leaks, not in the oil, just gone. I hear some grinding noises behind the dash sometimes too. Any other ideas on how to get this beauty runnin or ideas to the cause of the prob would be great too.