The very best way to do it os to look for recommendations. Are there anybody you know locally who is learning/has just learnt?
Other than that take a few lessons with out commiting to a very big prepayment before you are comfortable with the person and style of tuition.
If you don't like an instructir just give it a break and book with someone else. ADIs are people and you do sometimes find a clash of personality...they are only human.
Go for someone mid-priced. If they are expensive it doesn't necessarily mean they are great - but if they are cheap they probably need the business.
National schools will always be most expensive as the instructors have large overheads. It doesn't necessarily mean that the instructors are any better - they all have the same qualifications. They will probably have a series of polcies to adhere to - some for the benefit of the pupil some for the benefit of the company.
With a school you have a support network of other instructors should you insrtuctor have a mechanical breakdown etc. With an independant instructor they are more likely to be good as they have their own reputation to feed - BUT you don't have as much back up if mechanical failure. Having said that local instructors know each other and can help out but it is not guranteed.
Probably the most inportant thing is tht you get a fully qualified instructor. You will know by the fact that they display a greeb license in the window. Trainees don't charge less but obv have less experience.
WRT asking questions that are subjective - remember you just get an answer over the phone which can be twisted. Answers to "what is your pass rate?" should always be qualified by a statement such as "over 6m or 20 tests". You can be misled by an instructor who has a 100% pass rate but doesn't tell you that is over a run of 3 tests 3 years ago IYSWIM.
Probably the best subjective question you could ask is "how many of your peoples are recommendations (as opposed to cold calls)" - again can be difficult to know the answer is 100% truthful.
Most of all a busy instructor that sqeezes you in/makes you wait 2 weeks is a good sign.
HTH (DP is a driving instructor).