Attend a class
As inconvenient as it may be, your best bet is to show up and observe or participate in a class. Almost every legitimate martial arts school I've visited has allowed new students to try out at least one class for free, and all of them allow you to observe general sessions with very little hassle. We are not in the era of "secret dojo techniques" anymore, and you should be wary of any school that claims the same. Observing a class will let you know what the focus of the school is, and what you might expect.
Caveats to that statement
Some schools do not allow you to jump into a general class for liability or safety reasons. If you show up on the day that the class is practicing neck cranks, the instructor probably will not want to risk a new student injuring themselves, or someone else, by reacting the wrong way to a technique. If it is a weapons class, they may not want to put a blade or chain weapon in the hands of someone who may injure themselves or others. Many schools will give you a one-on-one training of basics instead.
Individual instruction is different from group instruction
Having an instructor show you the techniques one-on-one can make things seem really easy, and engenders a feeling that the teacher truly cares for you as an individual. In actuality, you will likely be one among many, which is why you will want to insist on at least viewing a group class.
Watch the instructors, not the techniques
I know you probably want to see all of the whiz-bang things that you'll be learning, but more important for now is to see how the instructors comport themselves. Are they noticing when a student has trouble? How do they go about correcting an error? Are the students treated with dignity? Is training by instructors or by senior students? And, for your case, does it look like they're teaching practical self-defense? Is it fanciful movements that only work with a willing partner, or just calisthenics?
Above all, ask questions
This applies whether you see things you like or you don't like. Maybe you walked in on the day when they're doing conditioning for the upcoming tournament (as Sardathrion notes, conditioning is important for general health). Maybe the parent organization just changed syllabi, and that's why the instructor seemed uncertain of what technique came next. Maybe some other red flag came up for you and you want to ask a student or instructor whether this was a typical class. If people are wary to answer questions, be wary of them.