Hubspot advises on exit interview processes. 

Link: How to conduct an exit interview

Questions include:- 

• How long have you been considering leaving? 
• How did the job match expectations? 
• What reason primarily describes your reason for leaving? 
• If you have accepted a new position, what is most attractive about your role? 
• What did you like most about your job?
• What did you dislike about your job? What would you change about your job? 
• How would you describe the culture of our company?