Check your tax return. If you claimed depreciation (Capital Cost Allowance, or CCA), it will be on your tax return, and you should have a form where you kept track of how much you have claimed to date.
Depreciation is claimed, to generate losses / offset in rental income. It is a book keeping assumption that your property wears out over a set number (e.g., 25) years. It allows you to offset income gradually, and not get a large loss when you sell at the end of life. BUT When you sell for a gain, you need to claim back those income deductions previously taken.
If you never claimed CCA on prior tax returns, there is nothing to worry about.