A Tutoring Qualification
For a number of years we asked tutors to prepare and teach a lesson as part of the application process, and we used this to make a decision about which tutors to take on. However, tutors develop their skill as they tutor, and we want to encourage contintual development and a desire to improve among all of our tutors for as long as they're tutoring.
Tutoring Standards offer a professional tutoring qualification which objectively assesses a tutor's ability and gives constructive feedback on how to improve. Taking this qualfication is the best measure of tutoring ability that we know of, but it's also a great learning tool so that you can develop as a tutor.