How would you choose a Driving Instructor?
I would call around.
There would be some 'standard' boxes to tick:
1. Insurance
2. Permits to operate in a suitable learning area
3. Certification: IKO, BKSA, First Aid
4. Equipment: Recent and appropriate kite model, harness and board, radio helmets, safety gear - boat, lifevest.
Then there would be some more subjective measures to seperate those that can tick the above boxes:
1. Recommendations from your friends / forum users (or cautions!)
2. Have a chat with not just the boss but some of the instructors. Check out ongoing lessons. Good vibe??
3. Price / Packages / Location to you
4. Expected speed of progression eg, I've surfed / snowboarded / familiar with a trainer kite
Hope that helps! Have fun!