Actually, the heater cores aren't notorious for leaking or for failure, the heater hose connections under the hood are though.
Inspect them with a mirror and flashlight while under pressure. Look for any evidence that coolant is leaking and getting into the hole in the firewall.
If those connections are completely dry, look for coolant dripping from the AC condensate drain. If there is any, coolant is leaking from the core.
Also, it is possible to smell coolant in passenger compartment when the leak is anywhere under the hood since the HVAC system draws outside air through the cowling on the passenger side of the car.
Best bet is to pick up a pressure tester from AutoZone (AZ9300, $75 deposit) and pressure test the system. There are many potential sources for leaks in the plumbing.
Steve