Here’s a blurb from the Canada Borders Services website – you will need to drop into the Canadian Customs office and get them to issue a Form Y-38:
Protecting your valuables
Before travelling outside Canada with valuable items, you may wish to take advantage of a free identification service that is available at all CBSA offices. This service is available for items that have serial numbers or other unique markings. For items that do not have such markings, the CBSA can apply a sticker to them so that they can be identified for customs purposes as goods that are legally in Canada.
When you show your valuables to a border services officer and state that you acquired them in Canada or lawfully imported them, the officer will list your valuables and their serial numbers on a wallet-sized card called a Form Y38, Identification of Articles for Temporary Exportation. If you are questioned about your goods when you return to Canada, show your card to the officer. This will help identify the valuables that were in your possession before leaving the country.
Of course, you should have with you the paperwork to show the CBSA officer that you own the items. If the papers show that item was purchased outside of Canada, it'd be a good idea to have the receipts to show that all required tax and/or duty was paid, if that was the case (of course, many people won't have kept those sort of papers so your word might be all you have if the officer even asks about that).