If you're not charged tax beforehand, you'll be taxed in Canada.
Under the provisions of the Postal Imports Remission Order, if someone mails you an item worth CAN$20 or less, there is no duty or tax payable; and if it's marked as "gift" from a friend, then that threshold rises to $60.
When it gets to customs, you'll be taxed regularly (I *believe* this is 13% - for Ontario residents anyway - but there may be other rules depending on your province and the country of the import) for the amounts above that threshold. Then the Canadian carrier will tack on another $10~ customs handling fee.
Example: I got a gift valued at $70 from the US. They charged me 13% of $10, then I paid another $10 to Canada Post.
Hope this helps!