Agreat ScrumMaster could handle more than one team at a time. But if your organization is under transformation and is yet to reach a particular level of maturity in terms of Agile practices, each team should have an exclusive ScrumMaster to facilitate. Now onto the "Great Scrum Master" Checklist!
Your potential Scrum Master focuses on people. The person talks about helping people develop and giving them the opportunities on their projects to do so. The Scrum Master should understand each team members strengths, weaknesses, aspirations, and even fears in order to help the team optimize delivery.
The Scrum Master should try to align an individual's goals to a team's / product's shared goals whenever possible. He / She can only show the possible doors to the team member - it is up to the team member to walk through which one and how.
This is something really hard to gauge in an interview, but when you talk to someone you can feel their energy in their voice and in their body language. Do they sound passionate about Scrum and more importantly, people? They don't have to be an extrovert and loud-spoken.
Removing impediments for the team is a central day-to-day responsibility for the Scrum Master. Often times impediments come in many forms. Perhaps the team needs to coordinate a hand-off of a dependency. Scrum Master should help facilitate that conversation to make it happen.
It is up to the Scrum Master to shield the team away from "all the talk" and rumors and provide them a safe environment to execute as a team, learn as a team, and speak freely as a team.
Sometimes we hate the word, "Scrum Master". Particularly the Master part. Sure, they should have quite a bit of experience practicing a framework such as Scrum, but aren't supposed to have a traditional master-student relationship that we typically perceive what a "Master" should be.
The team can ask the Scrum Master for guidance, opinions, recommendations, and the Scrum Master can ask powerful questions to the team to get them thinking, but the Scrum Master should not be directing team members to do things.
Great Scrum Masters always listen. They only speak when necessary to guide and facilitate the team. Great Scrum Masters don't dominate conversations - but all of them understand the conversations and how it may affect the team. A great communicator is a given, but being a great listener is much harder.
This is more of a bonus but it really relates to energy and the work environment. Planning happy hours? CostCo runs? Having a survey to pick what snacks and equipment the team needs in order to operate most optimally? How about needing more nerf ammo? Laptop not working? Need to procure a software license?
Scrum Masters that really rock on their projects eventually are not necessary. This is the gold standard for Scrum Masters. I've never seen it, it's more like a unicorn. But I've seen some teams get awfully close. Great Scrum Masters can guide the team to self-organization nirvana to where the Scrum Master is just about out of a job.