you'd know if the heater core is leaking the inside of the car will smell bad of coolant.
And a head gasket can leak in so many places that usually oil and coolant don't mix together untill it gets real bad or just happened to be in that spot. Keep in mind that I've also seen them leak only one way at first. Meaning the cylinder can suck in some water but not blow hot gas into the coolant, and vise versa I've had a head gasket that only blew hot into the coolant not enough to overheat it while driving but enough to instantly boil it when you came to a stop.