Ask them how long it takes to get a black belt. If they say a year, it's a McDojo. If they say 3, but more likely 4 yrs, it's likely a far more reputable school. Get the specific name of the style of martial art and look it up online to see how prevalent it is.

I've done a ton of martial arts and have 2 black belts. Kung Fu is a lot of fun, but a little inefficient for self defense on its own, but in terms of building power and developing strong technique, it's invaluable. Karate is okay. I have a black belt but only cause my parents kept sending me there, I'm not the biggest fan of Karate.  Capoeira is a lot of fun too; good exercise.

End of the day the martial art I've been most impressed and happy with is ITF Taekwon-do. Although I absolutely stress ITF over WTF. WTF is quite aptly named IMO. WTF is the Olympic Taekwon-do and their sparring is an absolute joke throughout the martial arts community. They only kick which is a useless display of ability. Hands work very well, why cut them out? I have done WTF, freestyle and ITF. My own suggestion would be freestyle or ITF Taekwon-do. If you can find a club that does either of them, you'll be in good standing in an internationally recognised art.