Then how many years of aikido experience does one need to have before being able to evaluate a teacher?
After about one year of practice I had to move to another town and started at a new dojo.
It was horrible:
The new sensei being a higher rank than my old sensei wasn't as good, or knew as much.
He didn't know how to do kote gaeshi right.
He couldn't do proper ikkyo omote. Just a caricature.
He even talked while bowing!
Today I myself most often use both hands when doing kote gaeshi.
I explain and show my students why it is not very usefull to swing one's arm behind the hip when doing ikkyo omote.
And I myself say "onegai shimasu" or "domo arigato gozaimashita" while bowing.
And I know that the grades of the organization I started with and the grades of the one I belonged to after one year can't be compared.
Well, maybe about seven years?
And what advice would you give to people who want to begin training in aikido and have several dojos to choose from? How can they evaluate the teaches in those dojos?
They can't. They should go, just where the feeling is best.