While I'm not familiar with Canada's requirements, Mexico has an immigration law requiring a consent to travel letter for unacompanied minors or minors traveling with only one parent. The letter must be notarized by a notary public before departure and presented upon demand to immigration officers. This is how they reasonably determine that the letter is genuine. Interestingly enough, there is no requirement to provide the letter in Spanish. In the event that the "missing" parent is deceased or divorced, then a certified copy of the death certificate or divorce decree is substituted for the consent letter.

I once asked a immigration officer at MEX about the purpose of the law. He told me it was to help prevent incidents of international kidnapping, particularly in child custody cases. I suspect that this is also the reason for Canada's law and they would probably want a notarized letter from the absent spouse.