When interviewing, ask candidates how they have handled workplace problems, such as a conflict with a coworker or a difficult boss. Focus on specific projects or transactions, not just the work in general. Listen as the applicant talks about past workplace problems. Does he/she still seem angry or irritated? Does he talk disparagingly about coworkers?
You should also know the stressors inherent in the position for which the candidate is interviewing, and look for someone who has handled these successfully in the past.
Ask the candidate to sign a waiver allowing you to talk to his former employer and waiving any liability as a result of the conversation. Fax it to the reference prior to your conversation. Ask the employer for names of vendors or clients with whom you can talk.