I did have to fill the family information form even applying within Canada, but apparently not everyone gets asked to submit that one.I might be wrong but there was questionnaire that we filled out before submitting the documents and one of them is asking where you are applying from? I think if you apply pgwp outside of Canada you have to submit more documents than inland applicants like the family information form.
After some googling, it seems applicants outside of Canada apply using IMM1295 instead of IMM5710, so I guess that's how they know.