Three things that I would look for are basic swing knowledge, experience in teaching, and communication skills. The last one may be the most important of all assuming a sound knowledge of the first two.
In my experience word of mouth and referrals have provided me with more students than any advertising videos marketing gimmicks, etc.
Usually the first lesson will Supply enough information to make a determination whether to proceed with this instructor or not. A good instructor can tell you 15 ways how to do one thing and that usually comes from experience.
Don’t be afraid to ask a lot of questions in your first lesson And good luck finding the best instructor you can.