For diagnosis, my impression for adults is that it's usually in effect a self-diagnosis with the doctor rubber-stamping the request. Some doctors are probably more free with it than others, but I haven't heard of a situation where it was more strenuous a screening than asking a series of questions, where it was very obvious which answers were the "yes, I have ADHD" answers. So basically if you believe that you have ADHD, you can get yourself a diagnosis for it.