Check their years of experience teaching or instructing

Why? For 2 reasons.  

Firstly after 20 or 30 years or more they should have accumulated considerable knowledge over that time.

Secondly, if they have been teaching for a long time it shows they have been consistantly helping others and that their students have stayed with them because they like the instructor/s. They wouldn’t hang around them for long unless there is some good benefit to the student.