With all of the information available on the internet nowadays, all of the people sharing their own personal stories about why they transitioned, being able to compare symptoms, compare life experiences, and test and test your own convictions on the matter based on those stories and experiences, I think it's pretty safe to say that someone with classic gender dysphoria could probably diagnose themselves pretty easily, and in fact most of us go into transition already knowing that we need to transition, need hormones, and know whether we need surgery or not. There are plenty who are more on the fence about it who probably could benefit from some guidance to sort out their doubts and uncertainties, but most of us, probably not.
I basically did diagnose myself. A therapist was very helpful once I finally did start going to therapy, because she helped me sort out which of my life problems were likely dysphoria-related and which ones were likely general-life-issues related, but she was more help in getting through the actual process, not so much in helping me decide what I needed to do medically. I knew long before going to therapy for the first time that I was indeed trans, and that I wanted hormones, a full social transition, and SRS. She just confirmed what I already knew, and helped me rest comfortably in those convictions because I knew that a medical professional agreed with me on them.
