There are many sources for background checks. My question to you is has your mother thought about hiring a property manager? Property manager typically charge a fee, but they typically make sure that fair housing rules are followed, conduct tenant screenings, check references, and handle tenant issues. I would advise her to being with that. If she decides on going it on own, check out the National Tenant Network. Regardless what she does, she will need to make sure she gets written authorization from the applicant and she follows the same process for all applicants -- no exceptions.