I concur with the questions submitted by Eric and Heike.
As a gym owner too, I like to see how interviewees handle role playing scenarios. Put them on the spot to see various customer service situations. You can also use other staff members to help with the role playing. If the interviewee is new to the gym environment, they may not know how to handle some of the intense situations, but it is important to see if they can think on their feet.
Also, ask if they have any sales experience. Everyone in the fitness environment should have some sales experience or at least be able to recognize when a customer has the desire to purchase and then get someone to help with closing.
I hope this helps. I wrote a blog post on how to handle various gym member complaints. It may help with brainstorming other questions. http://blog.gyminsight.com/2013/10/how-to-handle-gym-member-complaints/
Best in Health,