Old brown breadbox type with black-ish keys C64: always 6581
White "modern" style C64C: depends, you have to look underneath, there's a ventilation grill at the front of the machine, if you see the motherboard through it, it's an old type motherboard, which means it has a 6581. If you don' see the motherboard, it has an 8580. The newer motherboard has smaller footprint and this is why you can't see it through the grill.
Old brown style but with cream keys C64G (you probably won't find this in Canada): has 8580