I think this would still apply to Subaru's - If the head gasket has gone you will find white sludge in the oil, check inside the oil filler cap for white sludge, also, oil can get mixed in with the coolant if the gasket has gone so check inside the coolant resevoir for oil floating on the surface.
Personally I do not believe that your head gasket has failed, is it a brand new radiator you had fitted?.