If you were born in Canada, but no longer live there, then your flag should be either India or Brasil
How does one determine their country "allegiance"?

Unless you are playing in an Olympiad or something, picking a flag is meaningless. You represent nobody.

Thanks, very helpful. I was wondering if there was such a thing as choosing the "Incorrect" flag in chess federations.

If humans ever represent the U.S. in anything but a hot dog eating contest then it would be an incorrect flag

Look in your fridge. Moosehead or Budweiser?
Actually, talking about American/Canadian citizens, there are a couple tribes which cross the border. The site should probably add an Iroquois flag. I know there's the occasional dust-up over that issue in other sports.
I was born in Canada originally but moved to California at 8 years old. Does that mean I allign with Canada, or America? (I don't play in an official chess federation so I don't know).