Finding the right Martial Arts dojo can be a frustrating experience.

The only reliable way to find out if a dojo is any good is to check it out in person. Don't be fooled by fancy appearances or lack thereof. A dojo isn't made of walls and furnishings and decorations, it's made of people.
There are many styles of Martial Arts, many regional and international organizations, and many dojos that are more or less independent. A dojo's stylistic or organizational affiliation is entirely irrelevant to your search.

A dojo is made of people, and within any style or organization, there are people who are nice to practice with and people who are not so nice to practice with. There is no "One True Martial Art (of any style)," and there is no teacher or headquarters or organization that is the ultimate authority in the world of Martial Arts.
There are, however, a number of folks out there who believe that their Martial Art is the One True Martial Art and that they or their teacher or their organization are the one rightful and central authority in the world of Martial Arts. Avoid these instructors - no "way" is the "one true way".

Ask the instructor or a senior student if their dojo is affiliated with a particular style or organization, and then ask them what distinguishes their style from some of the other ones in town. If you get a thoughtful explanation, good. If you get an evangelical sermon about why this style is right and all the other ones are wrong, excuse yourself politely and leave.
Because there's no central authority in the Martial Arts world, the numbered ranks and titles of black belts don't actually mean anything outside of the hierarchy of their own organizations. You might suppose that if an instructor has a 7th degree black belt, it means that that instructor is better than an instructor with a 3rd degree black belt, or even a brown belt. Not so! It's just as likely to mean that the instructors in question are from different schools, and one school gives out high ranks more readily than the other. Or that the 3rd degree black belt is too busy paying attention to her students to bother with advancement in her organization's political hierarchy.

For that matter, a person with no training at all can buy a black belt in a store, declare himself an instructor, and grant himself a 10th degree black belt! There's no law against it, any more than there's a law against someone who's never painted before declaring himself the world's greatest painter. The moral of all this being: beware of schools and instructors that try to impress you with rank, because all that those numbers say for sure about an instructor is how much of a big shot he or she is within his or her own particular organization.
What we're getting at here is that there's a lot of impressive-sounding rhetoric out there that really just amounts to flashy advertising designed to impress the uninformed consumer, and get you to sign on the dotted line. Statements like, "Our teacher is a Grand Master" (a term that always should raise suspicion), or "a 10th degree black belt trained in the One True (insert style here) World Headquarters" are really no different in intent or validity from statements like, "Our brand of deodorant is used by this popular television actor." In a really good dojo, the instructor and senior students won't be trying to impress you with this sort of slick marketing - they'll be too busy making sure that you have a safe and enjoyable first lesson.

Because a dojo is made of people, the only way to really know what kind of place it is is to look at the people. Look at the instructor and the more advanced students. Do they look happy and relaxed? Do they look like kind, good-hearted people? Would you want to move the way the advanced instructors move? Aikido in particular is an art with great power to transform people, so have a good look at what you might end up being transformed into. How do the people in the dojo relate to one another? A dojo is always going to be more formal and regimented than, say, a belly-dancing class. But are there smiles? Is there room for fun? Especially stay away from places where the students appear to be frightened of the instructor.
Try places until you find the right one. Be warned, though: learning aikido is difficult, confusing, and often frustrating - and, because aikido directly addresses our relationships with fear, aggression, and personal boundaries, good aikido training is guaranteed to push all of your least favorite buttons sooner or later. So even in the best dojo (especially in the best dojo), practice will sometimes feel all wrong. It takes great willpower, courage, and patience to stick with aikido training, and these are things that no dojo and no instructor, no matter how good, can provide you with - you have to find them within yourself.
You don't need to sit on the sidelines and watch a class first. Martial training requires a bit of courage. If you're serious about learning Martial Arts, start exercising your courage right away.

Get on the mat!