Always call current and previous landlords or managers for references--even if you have a written letter of reference from a previous landlord. Be sure to ask questions about the following issues:
Also, contact an applicant's employer to verify income and length of employment. Some employers may require written authorization from the employee before talking with you; rental applications usually include this type of release.
Take notes of conversations with all landlord, employer, and other references and keep your notes on file with rental applications. You may need this information should a disappointed applicant file a discrimination complaint or lawsuit against you.