Firstly, ensure they are a bone fide driving instructor. All driving instructors are regulated by the DSA so they should have a green badge showing they are allowed to teach for money. A red badge is a trainee which doesn’t mean they aren’t any good, just that they are in training and being remotely supervised by a qualified instructor.
I would choose based on the pupils ability to build a relationship with them. You need to be relaxed, feel you can make mistakes and not be embarrassed to swear etc. when you do. Recommedation is probably the best way for initial selection, but don’t be afraid to bin them if the relationship doesn’t work.
I also think the “pass fast” type of instructors should be avoided. The test is just one stage of really learning to drive. A few lessons around test routes and teaching tricks to pass is not proper instruction. Not to put too fine a point on it, but the real objective is to give the pupil the skills to keep them alive (and their passengers) once they are out on their own. This requires time, practice and a wide range of driving experiences in different places and conditions. It’s not a cheap option but what price the life of your child? My mantra as an ADI was “it isn’t the first to pass that’s the winner, it’s the last to have an accident”.
Rich.