After reading ALs thread I can verify that the entire testing process is quite complex. I saw three therapists before settling on the third, and am considered still to be in the 'assessment phase' of therapy in the funded system that I am in.
We have had 8 session for the assessment phase, and at the outset my t told me she would do some informal assessment, where she found out information she required just through our general conversations, and formal assessment, where I would need to fill out various questionnaires. So while I am not in the US the process seems similar to here.
I had already filled out several questionnaires about anxiety and depressive symptoms before coming to this T (I saw a different T several times before asking for a referral to someone more experienced with dissociative disorders) and these had been passed onto the new T.
There were times when it was very easy to tell that the questions she was asking in 'general conversation' were assessment related. And there were likely many times when I had no idea as well. We spent a total of about 1 and a half sessions devoted to filling out forms. They were very similar to the questions asked on the site that AL posted - related to PTSD, sleeping issues, anxiety, depression etc. Some of the questionnaires asked about symptoms over the last two weeks, and others asked about symptoms in the last two (six? can't remember) months. We also tried to do the DES but had to stop that as we switched a couple of times during it, and she said we'd come back to it another time when we weren't so triggered. We never did get back to it to finish it though.
Part of the overall assessment also includes pulling in information from previous physical and mental health records (there is nothing about DID in there, although there is extensive cognitive testing). I gave permission for her to get information from my previous therapist in another country, who confirmed our previous diagnosis of DID with the DDIS (and her personal experience of us).

The final step in this assessment phase is the assessment with the psychiatrist, which is set for when I return home after my study. This is the part where they are formally testing for DID. I am guessing it will be in the form of a subjective questionnaire for me to fill out and then an interview? but I don't really know. I don't know the person who is doing it and I don't know what instrument - if any - she will be using.
I don't tolerate not knowing things well!
But I can definitely attest to the process being extensive and drawn out.