zkzkz is correct - because of the tax treaty between Canada and the US would file an income tax return as a "deemed non-resident". The test for customs duties is of course different and no matter what tax return was filed the question is one of the "settled routine".

Before returning one also needs to consider health care coverage. There may be a waiting period pefore you are covered by provincial health care, even if you paid Federal and Ontario income tax and a health care premium tax.

Are you sure that you want to come back to Canada?