If you get a coolant smell inside the car, and, or, the windows fog up in the defrost position, or there is moisture blowing out the heater vents, the core has a leak.
On my newly acquired 89, the hoses were bypassed when I bought it, so I pulled the hose stubs off the heater inlet/outlet, and capped one. I had my wife blow air into the other one with my air compressor. It had a massive leak that you could hear and feel the air leaking out of the heater vents in the car! It was coated in stop leak when I pulled it apart.