--Experience/Resume/Qualifications (documented, and this would include reputation)
--Skill/Ability to transfer knowledge (demonstrated)
--Integrity and Modesty (more difficult to quantify, but if you have some experience and ability to evaluate people...not all that difficult to recognize)
The ability to instruct is a special skill, and somewhat transcendant...if you are a good instructor pilot and a competent shooter, you will probably be a better-than-average firearms instructor. If you have a decent grasp of algebra and Plane Geometry, you will be a pretty good Math teacher, even if you have never stepped inside a classroom (other than as a student).
All those people who say that "those that can, do...and those that can't, teach"? I am not a teacher or instructor of any kind, except in the most informal definition of the word...but I have known a bunch of individuals who were excellent "do-ers" and at the same time, very competent instructors.