Word of mouth and gut instinct. Bhs qualifications are one of my last considerations, and I say that as a qualified instructor, but i rarely tell people that! In 10+ years of teaching no one has ever enquired as to my qualifications, they come to me from my experience in certain fields. And fwiw I have lessons myself from people with no qualifications, one being a young girl with no bhs exams to her name but a great feel and empathy for horses, sometimes her communication and technical lingo is lacking but I get far more from sessions with her than a lot of others!!