I can’t tell anything definitive by the plugs, myself. Make sure the radiator isn’t leaking around the end tanks (where the plastic meets the alum). I’ve had those leak slightly for a pretty good while before I found the leak. GM did had some problems with certain head castings cracking. Would probably be evident by removing the valve cover.
You might consider bypassing the heater core for a while and watching the coolant level. If the level still drops with it bypassed, you don’t have to tear the dash apart to get to the heater core.