Hiring a Scrum Master is a Hard Thing to do.
There are over 400,000 Certified Scrum Masters in the world. Most of them are just Scrum Master on Paper. I help compaines screen candidates frequently I recently contributed to a blog post on Luis Goncalves´Blog on the topic the scrum master interview questions here. It is a great list of questions.
If you don’t know who Luis is you should check him out.