You don't need to wait until an employee leaves to hold an exit interview. You can gain valuable feedback by asking a few key questions of candidates who turned down your job offers.
Use typical exit interview questions that focus on compensation, managerial style and work environment, such as: What is your impression of our work environment and culture? Did the compensation package meet your expectations? How could it have improved? What was the deciding factor in your decision to reject our offer?
Some tips: Dig below the surface. If candidates say they received a "better offer," find out what that means: A better base salary? Better benefits? A flexible schedule?
Ask about the interview process itself. Was the interviewer prepared? Did the person receive mixed messages about the job or the company? Also, ask how candidates' expectations of the company measured against the reality they found when interviewing. Their answers can reveal a lot about your reputation in the job market.