Choosing a good instructor can be the most important decision a novice may ever make. Ask about the instructor's rank and membership in recognized martial arts organizations. You should ask how many members are in that particular organization. Another good question is whether there are other schools also certified within the same organization, especially if you may be moving soon or frequently travel. Frequently, legitimat e organizations sanctioned by the All-Japan Kendo Federation, International Judo Federation, etc., will confirm an instructor or school's credentials.
Word of mouth is usually a good method of finding a qualified instructor.
If possible, ask a friend or someone you trust to recommend a good teacher. Attend some of their regularly scheduled classes to see how they interact with the students. Avoid an y instructor who is overly concerned with discipline or abuses their students, either physically or verbally. Most importantly, trust your own instincts. A good instructor should not ask you to do any thing you're not comfortable with or physically capable of.