An interview method whose acronym is "DISCO" is used -- they ask extensive questions and I assume they are trained to discern consistency or inconsistency of answers, to dig deep and frame questions in a varying ways due to the fact that the person being diagnosed is necessarily having to self report their life history. I had this method and I'm in the UK. I also brought along a detailed (I mean ridiculously detailed!) list of all traits I believed I had and have historical and to the present, categorized by type and by age period. I wrote down every memory I've ever continuously carried with me that is still with me in the first place because it was a "What's wrong with me?" moment, from my earliest memories onward, as I have a vivid memory that goes back to a very young age. I also brought old family photos, literally the actual physical old 1960s items, not digital or anything that could be faked -- some of what I have couldn't look more spectrummy if you were making a Hollywood movie about a kid on the spectrum and fabricating "awkward family photos 101", lol. Seriously.
It's probably much closer to ideal to be able to bring along either a family member or a friend who has known you fro the earliest age possible, to verify early behaviours, as diagnosis is based on criteria that has to be present and evident in childhood, not just suddenly recently (in which case whatever the person is experiencing is probably something else).
The ideal is another person's testimony but you can be diagnosed as an adult all alone, you just have to try to think of, write down or bring everything you can to the assesments. The clinician will take care of the rest in the form of extracting from you a fuller picture, including questions about any historic well-functioning and not just your poor functioning, other diagnostic history, and they should also screen for other conditions or disorders in order to effectively rule those out. This is my experience in the UK.