Heater core you usually will have water/antifreeze leaking on the passenger side, fogging up of the windshield when you turn the defroster on, or a sweet syrup type smell usually accompanied by some smoke coming from the vents.
If the cats are plugged, junked, you may smell rotten eggs when the car is running, or they may glow bright red after you drive it a little then stop.
From my heater core problems, I usually smelled the sweet smell of the antifreeze when the heat was on. Also, vapors would come out of the vents and leave a film on the windshield. Thats how I knew the heater core was shot. ALso, with the cats you can hear them, they start rattling because a piece of them broke off, and just like SGARLIC said, they smell like rotten eggs.
Until you get the heater core fixed, bypass the hoses so they do not go into your firewall. I would also suggest getting a stem vac or something to try and get rid of the smell, maybe some carpet cleaner too.
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.