For dissociative identity disorder (DID) to develop, there is usually chronic trauma in early childhood along with significant problems in the child-parent relationship.
Diagnosis of dissociative disorders is by no means straightforward, mainly due to a lack of training and knowledge. The main diagnostic criteria can be found in the DSM-5 and this article explains how diagnosis is made.
There are a number of diagnostic tools available for assessing dissociative disorders. This article lists the principle ones.
Dissociative disorders appear as diagnostic categorisations in both the American-based DSM-5 produced by the American Psychological Association (APA, 2013), and the other ‘diagnostic’ bible used more widely in Europe, the World Health Organisation’s ICD-10.
DDNOS is seen by many people as a ‘not yet’ or ‘a not quite’ version of dissociative identity disorder and although it is supposed to be a ‘residual category’ and only given to a few people, in fact the vast majority of people diagnosed with a dissociative disorder fall into this category.