Good article! Finding a good riding istructor is the best things you can do! I also recommend finding a barn where you can tack up you own horse and where you'll have a chance to learn about stable mangement. These are really important to learn whether you are planning on owning a horse or not.
Other qualifications I would ask about- Do the trainers carry insurance? Is there rhyme or reason to the lesson - is it really preparing you for potential future ownership or are you showing up to a tacked up horse and riding for an hour?
You can find good instructors and equine facilities that have gone through an intensive certification and accreditation program at http://www.CHAinstructors.com