Check they have a direct link to the masters of their Style
Many of the best Martial Arts schools or best Karate Schools have historical roots that go back centuries. Where the knowledge was passed down from master to student for many generations. Each generation trying to improve on the great knowledge and skill of their master before them, practicing almost every day over the course of their life time. It wasn’t a case of ‘I’ll try this for a year then I’ll do another sport’, you did it throughout your life always aiming for perfection of your skills and knowledge over your life time. In short, if you want the real knowledge and skills no matter what style of Martial Art you do look for a style that goes back to the country of it’s origin.