You're about to place yourself in the control of a stranger. Make the instructor earn your confidence. If you sense the Instructor is offended by your questions, perhaps it's time to continue your search. It's also perfectly acceptable to ask questions regarding the history of the art. If the Instructor is vague or elusive in his response, beware.
Martial artists are notoriously proud of their art and it's heritage.
A credible Instructor will be more than happy to provide you with the lineage of the art. Pay close attention to the other students. A simple fact is that many martial arts will favor a specific body type over another. Arts that emphasize strong kicking techniques tend to favor slim, long limbed body types. Arts that favor grappling or hand techniques tend to favor those with good upper body strength.