You can now check the CRA’s website to see whether a person really is a GST registrant. The CRA explains:
The GST/HST Registry lets you validate the GST/HST number of a business, which helps to ensure that claims submitted for input tax credits only include GST/HST charged by suppliers who are registered for GST/HST. You can also use the GST/HST Registry to verify GST/HST registration for other purposes.
For example, a vendor of real property can check whether the purchaser is registered so that the vendor need not collect GST on the sale.