The psychometric questionnaires
The questionnaires: before the first session, or shortly after it.
I set you up with an account in the client area and send a small pack: general wellbeing, mood, anxiety. These are not about the diagnosis, they are about how you are now, and the period after a late identification is often harder than people expect.
Then, if it makes sense, the profile part. With late diagnosis one of them is unlike the rest: a retrospective questionnaire, about childhood. It seems strange at forty, but it is often the most useful, because it puts into words exactly what nobody saw at the time. A few people have cried while filling it in, and that is not a bad sign.
You fill them in on your phone or your computer, at your own pace, stopping halfway and coming back tomorrow. The font can change, the automatic jump to the next question can be turned off.
It is not an exam and it does not replace a formal assessment. What it does here is something else: it gives you a written form of what you have lived, which holds when the doubt comes back, because the doubt does come back, especially after a conversation with someone who does not believe you. You see the results too, in your own account, and you can take them with you to an assessment.