According to About.com, when checking references, ask questions that address the potential employer's concerns, such as personality, productivity and overall assessment of the candidate. Also verify that the candidate provided honest answers about his start and end dates, salary information, reason for leaving, key responsibilities, management roles, relationship with co-workers, and strengths and weaknesses. Questions that reveal the candidate's character and work ethic are important as well.
About.com advises asking the reference to describe his reporting relationship to the candidate. This question is to find out if the reference was the candidate's supervisor, coworker or another working relation. Doing so provides information regarding how the reference observed the candidate's work and interacted with the candidate. Also ask questions about the candidate's outlook and attitude to discover the candidate's work personality. Be sure to also ask about the candidate's commitment to quality and his approach to customers.
Monster.com recommends asking how long the reference worked with the candidate. Ask what the reference feels the candidate needs to improve to further his or her professional development. Also ask why the candidate left the company and if the candidate could have stayed if he or she had wished. Think about the information regarding the candidate most important for the company to know, and then decide which questions to include before calling the reference.