Heater core test ?

I would just install a new one. I know it's a PITA, but compared to newer cars, it's cake. Heater cores are easy to get for that car & fairly cheap too. (RockAuto)
New aluminum cores are lighter & way more efficient than the old copper ones & it's just not worth the effort to check it if you're unsure of its condition. If it leaks, you'll have to replace it anyway, IMHO.