Edited excerpt from 3 Essential Steps to Doing a Thorough Reference Check by Jeff Markowitz:
Ask open-ended questions and get specific examples (e.g., “How would you describe the person’s leadership skills?”), not yes or no questions. Most importantly, don’t settle for vague answers such as “He/she is an experienced leader.” If you get an answer like that, ask the reference for an example of the candidate’s leadership skills in a specific situation. If the reference can’t think of one, then you should seriously question the relevancy of the reference.
As much as possible, always seek specific information about the candidate’s work and actions rather than generalities.
(1) “Ask open-ended questions” — this is one of the principles of asking questions generally. See How to ask great questions and Don’t ask multiple choice questions.
(2) “Always seek specific information about the candidate’s work and actions” — compare this to best practices in interviewing, in How to run a job interview.