A lot of good and bad info here. Mostly anecdotal, like "if the teacher is young yet holds a high rank in more than one martial art". That's just absurd as it requires people to make snap judgments based solely on appearance. I'm 44 years old and I still get proofed for cigarettes, people don't assume I'm older than 20 years. I hold a black belt in judo, I've participated in the AM-CAN eight years in a row. I hold a black belt in Tae Kwon do and for the past two years I've practiced boxing and Brazilian Jujitsu. I don't own a school and I am not a paid instructor. But my knowledge and experience pales in comparison to my Tae Kwon Do instructor who is 4 years younger than I am. Age, or the lack of does not predicate a failing as an instructor.
Signs that you're at an awful money hungry McDojo:
More than half the class holds high level belts with less than 3 years of formal training.
The school is less than 5 years old with more than 12 black belts. (12 being an average class size and 5 years being an extremely short -albeit possible- time to earn a black belt.
Contract payment method. If your school requires a contract of payment to learn, get out. An annual membership is not the same as a contract.
The school lead instructor (not always the owner) claims to have multiple belts in multiple disciplines but does not have proof of graduation or hasn't learned from a reputed master.
Belts are awarded and not earned. Belts should never be given without a very specific test for said belt. There is a tradition where a master can bestow a belt unto a student who shows exceptional understanding of the art without tradotional belt testing,but this isn't done in a class sething, ever.
If belts are given based on time spent or classes attended and/or lessons are individually charged. Belt testing is a must and if an entire class can receive a belt simply for attendance and not have to demonstrate an actual understanding of the art, you're in the wrong place.
Not everyone learns at the same pace, individual testing for belt/rank advancement is a must and taking the test is usually left up to the students discretion. There are exceptions and it's the masters job to push the students.
If you walk into a school and there are "kids" belts. This is a hallmark of an awful school and has unfortunately (or fortunately for those looking to spot bullshit schools) has become something of a trend. Kids classes will advance students faster through belt rank with a simplified class syllabus in order for kids to feel good and parents to feel like they are getting their monies worth. This usually means your school has no understanding of ANY martial art and it's students are going to fail at life. Often.