What has made the most difference to me is having experience with clients and in the early years in my career learning the ropes in agency work. I had individual (1x a week) and group supervision(2x a week). I continue to avail myself of consultation on a regular basis. I think being supervised in the work I did was the strongest evidence of my competence. Some of the credentialing that existed before the present testing involved getting written recommendations from clinicians who had witnessed the work I did. I did take one essay exam for a Diplomate in Clinical Social Work in 1999 and having to write out my thinking and have 3 other Diplomates read it and score it also seemed an in depth measure of what I can do. I don't think a multiple choice test is the best measure of competence in the field - it shows who can take a test well though.