Not all dojos are created equally though and at a glance they may all seem to look the same. Here are some things to look out for.
These are dojos in which gaining promotions is both very easy (in fact, some places just require a certain number of months of attendance, not necessary any gain in skill), and very expensive. In many of these dojos there are formal "tests" which parents pay for. Essentially the sensei wont consider them for a promotion without this payment, and as your rank increases so does the price. I've heard of students paying over $1000 for their blackbelt test. As a sensei myself, I understand the need to make money. Unfortunately, running a dojo is expensive between insurance, travel expenses and everything else. These belt factories take it to a new level. Frankly I see it as little more than extortion and giving students a false sense of their ability.
You may think that the villains of the original Karate Kid movie is fiction but unfortunately these people are out there and they're more common than you'd expect.
If you encounter a dojo that even remotely resembles this, I implore you to run the other way as quickly as possible. These are often very militaristic. They will say that "your child will learn discipline."
The discipline learned in true karate is having discipline over one's self, not being dominated weekly by an emotionally abusive sensei.
A dojo should be a family. It should not be boot camp for kids!
You don't need a college degree to become a sensei. There is really no governing system over who does or does not teach karate. As a result there is very little quality control. Here are some things to look for in a sensei who shouldn't be...
Does the sensei constantly strive to improve his/her skill? All too often we see someone earn their black belt, think they've "finished training" and then go on to open a dojo without ever working to learn more. Trust me, earning your black belt is not the end of your training, and if you think it is then you've missed the point.
Is the sensei 50+ pounds overweight? I respect people of all shapes, sizes, etc. but a sensei should be an example of healthy training/living for students. In our style we use the term "life protection" rather than "self defense." This is because a good sensei will teach you to protect yourself from a lot more than that rare occurrence that you are physically attacked. They'll teach you to protect yourself from heart disease, obesity, etc.
Does the sensei participate in class? This may seem like a silly question, but there are so many sensei out there who simply stand in the corner barking orders while students and "assistant instructors" do everything. Even Osensei, who is older than my grandmother still participates in every class that he teaches.
Also, find out about how knowledgeable the sensei actually is. There are far too many people out there who fool dozens of students and their families by being able to simply throw a few high kicks.