The best tip is to follow the advice published on http://www.musicteachers.co.uk
There is also a search facility on the site to find music teachers.
This organisation works in conjunction with the Musicians Union. For the protection of its members, the MU publishes the names of individuals and organisations where it feels members should receive advice and information before proceeding with any involvement: please see p 33 of http://issuu.com/musicians_union/docs/the_musician__autumn_2012
The MU only advises its members, not the public. However, it is unlikely that an individual or organisation that is a cause of concern to the MU would be listed on the Music Teachers website.
There will also be many GOOD teachers who are NOT listed on the Music Teachers website – just because a name is missing, do not assume that there is any cause for concern.
Use this advice published on the Music Teachers site to inform your judgement: