I've interviewed a number of candidates for salaried positions. I was recently informed that we are hiring a consultant for a few months to help us develop a native iPhone application. We are a .NET shop and none of us have any experience with iPhone development. I've also heard rumors that we are planning to hire more consultants in the future to help develop on other various platforms.

How should I interview these consultants, considering that I have no experience with this type of development? Without regards to platform specific questions, what are the things I need to keep in mind when interviewing consultants?

Normally when I interview candidates, I'll ask questions that I know the answer to. I would never ask a candidate something that I don't know how to answer. Since this is a different situation, what do I need to ask in order to make sure they are subject matter experts in their respective fields? Is it appropriate to ask specific questions that we'll need to eventually solve during the interview?