Ask them on their records:

Ask the potential boss about their experience, how long he or she has been at the company at the same position and where he or she had worked before they joined the present company. Ask them reasons why the former employees left the position. If there is lot of turnover in a short amount of time, or if the interview panel is uncomfortable answering the questions, it can be an indication of there could be problems with the boss.

Caren Goldberg, Ph.D, is an HR Professor at the Kogod School of Business at American University. She recommends the people searching the site eBoss Watch, where you can read reviews that former employees have given to a boss. And if you are serious about the position, she also suggests you to reach former employee whose spot you are interviewing for, and ask them on their opinion on that workplace.