If you are an employer, you can verify your employees social insurance numbers by calling Service Canada: Section 3 - Employers and the Social Insurance Number
Employers may speak with Service Canada’s Social Insurance Registration office at 1-800-206-7218 to verify the SIN assigned to their present or former employees. You will be required to provide your business number (issued by Canada Revenue Agency), as well as appropriate identification for the company and the SIN holder.
Unfortunately, people forging SINs now have the formula used to make a technically 'legit' SIN.