Bring the candidate scrum master to the team stand up and have them listen in with you. Afterwards, simply ask them how many impediments they heard come up during the stand up. If they didn’t hear any impediments, then they probably aren’t a good candidate (hint: no impediments from a team is an impediment all by itself). If they heard more than one impediment, then perhaps they are a decent candidate.
Want to take it even further? Pair up for the remainder of the day and chase one impediment down together. If you nail it by the end of the day, then share a high five and hire that candidate! For a prospective scrum master, dealing with a real organizational impediment will tell them more about a company’s culture in one day than attending a week’s worth of meetings.