I was diagnosed by a psychologist who asked me to do a whole bunch of tests, some were fill in the bubbles, some were finishing sentences, some were ranking on a scale, some were yes/no. Then I had a several hour long interview, then he wrote up a report. I would say that the whole thing took 6 hours or so.
I am in the US, but at least in this area, psychiatrists do not usually diagnose DID. I was sent for testing by 2 psychiatrists. They suspected, but didn't want to "saddle" me with that diagnosis. The psychologist that I saw specialized in testing/diagnosing people who had a history of trauma. He also diagnosed MDD (which was clear to everybody) and mixed personality disorder (which my Ts think is more due to the DID showing through).