My initial diagnosis came from a social worker in 2017. I questioned it, not believing that it was possible. She referred me to a psychologist who came to the same conclusion. Then I saw a psychologist through the VA (PhD level). He came to the same conclusion. My question to him was, Is there some kind of test that can prove this?
He said there was. Start on HRT for 30 days. If you feel like anything is wrong, stop immediately, and we will look for a different answer. After 30 days, I felt great and agreed to continue. He warned that after six months, changes become permanent, so pay close attention to how I feel about the changes that have started happening.
As Sarah mentioned, how you feel about the physical changes is a key indicator. Dysphoria can work in both directions. If you are not comfortable with the changes, or do not like what is happening to your body, those are dysphoria too and signal that this may not be the right path for you. Of course, I felt great and was happy with all that was happening, and never looked back. For me, it confirmed that I was on the right path and that the previous diagnoses were correct.