The best indicators I ever found were: Are the senior grades teaching the lower grades?
Is the instructor friendly and approachable?
Is everyone (from the top to the bottom) having fun?
Are there special fees, secret techniques, and a cult mentality?
Are many people injured?
Are all fees clearly labelled and explained?
What are the instructors' qualifications?
The dojo is not a McDojo .
With those you should not err too far.
Note that those are not only applicable to Aikido but especially the injury and qualifications ones are very applicable.
Aikido can be hard on the wrist, knees and ankles.
Break falls can be hard on your neck and back.
Thus it is important to know your limit and pay attention to instructions.
Note that qualifications includes both proficiency at Aikido (which can be hard to judge) as well as teaching qualifications and insurance.
As for styles, pick the right dojo.
Do not worry about the style as in the end, it is all Aikido.
martialarts.stackexchange.com/users/82/Sardathrion - against SE abuse
It doesn't matter what they say about the school.
It matters what they do.
I believe, the answer to the question lies in measuring the schools ability to help you find what you are looking for.
So go to the school and look for people who you can relate to.
If you can see people that you have something in common with and you can see people making progress along a path of change that would motivate you then this is a good school for you.
For example I heard once about this Aikido dojo that had a very impressive student body, they looked great.
For their warm up drills they did forward hand springs.
It turns out they were from a University and had been recruited from the gymnastics team.
What we are looking for is a place where we can make progress on our own path.
If you can see in the school the kind of change you are looking to make that school will probably work for you.
martialarts.stackexchange.com/users/923/kkron
Generally, it's pretty straight forward.
Go to a dojo (or contact someone beforehand if they a website and mail address) and ask if you can watch a class.
Watch the class.
Do not watch the techniques/movements/forms, but watch the attitudes of the teacher and the students.
Watch the interaction between them and between senior students and beginners.
Are many people smiling while working hard?
Particularly watch the teacher when he demonstrates to the students what to do, or when he intervenes between practising students, handing out feedback.
Does
he/she give advice/feedback or is he/
she just criticising or putting the students down?
Are there many injuries?
After watching, try to have a chat with the teacher (if he/she hasn't already approached you him-/herself).
Approachable?
Hopefully, yes.
When you leave the dojo, how do you feel?
Does it feel like something that you'd want to be part of?
Bingo!
If not, stay away.
You should feel a good connection first.
The rest, the movements/forms/etc, can be learned.
martialarts.stackexchange.com/users/6704/John Goverts
I wrote something a few years ago that is still fairly relevant.
The thing is - how do you choose a school, and that is not specific to aikido.
I hope this helps -- let me know if this brings up any questions in your mind.
The basic point is: make sure the teacher is personable and not just trying to sell you on a subscription.
Pay attention to the students.
How do the beginners look?
How do the intermediate students look?
The intermediate students should look like they have an idea what they're doing, and should look worse than the advanced students.
Finally - as a student, once you have chosen a school, you have a responsibility to shut up, listen and learn.
martialarts.stackexchange.com/users/68/Anon
It depends on what result you expect from studying martial arts, but in many cases it will be useful to look at the students there.
In most martial arts with sparring, students who are taught by a good trainer will win in competition.
I suggest instructors which have such students are better than those who have not.
In most cases in Aikido there is no such scale, but you can ask about certifications (both student and instructor).
Anyway, there must be some external verification of instructor quality available.
martialarts.stackexchange.com/users/91/Iarek
Try to consider the dojo as a whole.
Beside avoiding scams and cult staff you should look at this: the teacher has to be good and motivated.
the amount of talk from the students should be minimum or absent.
Basically you should hear breakfalls noise, not people speaking.
a healthy number of senior students ( one third of students is a good number)
white pajamas should be at least half of the total students.
(not necessarily true for very big dojos) students, especially white belts, should look very motivated.
there should be physical training.
If they don't look tired at the end of the lesson don't join the dojo.
Avoid dojos where senior students regularly give explanation to novices.
That's a break of etiquette and should be a seldom exception.
martialarts.stackexchange.com/users/10/tacone
What should I be aware of if I would like to start training Aikido? I used to train as teenager in small dojo. I picked up few tricks but I never got the essence of Aikido. Is there something you would recommend I research or be aware of before I signup with local dojo?
How to select the right Aikido dojo
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The best indicators I ever found were:   Are the senior grades teaching the lower grades?  Is the instructor friendly and approachable?  Is everyone (from the top to the bottom) having fun Are there special fees, secret techniques, and a cult mentality? What are the instructors' qualifications?   Go to a dojo and ask if you can watch a class. Do not watch the techniques/movements/forms, but watch the attitudes of the teacher and the students. In most martial arts with sparring, students who are taught by a good trainer will win in competition. Basically you should hear breakfalls noise, not people speaking. Aikido can be hard on the body and the most important indicator to be aware of before signing up to a Dojo,  is to see if the instructors are senior, qualified  and approachable.  
Be aware that Aikido can be hard on the body. There are a number of indicators to check before signing to a Dojo, the most important is to see if the instructors are senior, qualified  and approachable.  