Full size and intermediate size Pontiacs sold in Canada as new cars in the 1950s and 1960s used Chevy frames, suspensions and engines. I believe many of the cars sold in England and Australia were the same deal. So, if the car you're looking at has a straight 6 or 265ci Chevy engine, it's a Canadian car. If it has a 287ci Pontiac engine, it's a US car. If you see a '59 or '60 Canadian Pontiac, for instance, it's very apparent as they don't have the Wide-Track stance of a US car.
There's some more information available on Canadian Pontiacs at
http://www.canadianpontiac.ca/
or
http://www.bruneauperformance.ca/intro.html