It matters not whether that person is running a school from a hut on a remote island with lots of horses, or operates in the world's largest college. Seek out and interview your prospective instructor with all the care you would use in hiring a surgeon. Ask for references, not only from past students but also from horse owners who have used the school for shoeing. You would not be out of line to ask for the instructor's resume. He is, after all, your prospective employee.
Some farrier associations offer voluntary certification testing to their members.
In selecting a farrier instructor, check the level of certification the instructor has acquired. I suggest that you seek instructors who are American Farrier's Association certified journeyman farriers. By no means does it guarantee that an instructor is qualified, because a lot of good farriers are not certified at any level. Certification also does not tell you anything about the instructor's teaching ability, but it does give you some idea of the level of competence. Although a school's literature gives you an idea of what the school has to offer, don't finalize you decision without a visit.