There is the heater valve that is under the cowling between the firewall and the windshield. It is on the passenger side with two hoses and a red vacuum line going to it. You may be able to peek through the cowling with a flashlight to see if it is leaking. If you cant see it, then pulling the cowling up isn't too bad.

If your heater core was leaking I'd expect it to fog the windshield on defrost.