How is Hallucinogen Persistent Perception Disorder diagnosed?

Usually, HPPD is best diagnosed by a neurologist, as he knows the brain better than any other person.

The tests are mostly subjective, asking the patient what he sees etc... some MRI tests can see changes in the brain, while some can't.

In my case, my psychologist just took my word for it.
They have to rule out other disorders like schizophrenia or epileptic disorders in order to consider you a candidate for HPPD.
You must have visual disturbances that persist more than a few day post hallucinogen use.
Hallucinogen use must precede HPPD.

DSM-IV
A) The reexperiencing, following cessation of use of a hallucinogen, of one or more of the perceptual symptoms that were experienced while intoxicated with the hallucinogen (e.g., geometric hallucinations, false perceptions of movement in the peripheral visual fields, flashes of color, intensified colors, trails of images of moving objects, positive after-images, halos around objects, macropsia and micropsia)."
B) The symptoms in Criterion A cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.
C)The symptoms are not due to a general medical condition (e.g., anatomical lesions and infections of the brain, visual epilepsies) and are not better accounted from by other mental disorders (e.g., delirium, dementia, Schizophrenia) or hypnopompic hallucinations.

I have seen neurologists to check for brain tumors, epilepsy, traumatic brain injury and heavy metal toxicity. I have had two MRI's one EEG and various blood tests done. I went to a psychiatrist to see if it was schizophrenia, I had thought it must be since I was hallucinating. He said I did not have that but I tried the antipsychotics anyways which made my symptoms worse. He said it could be acute anxiety or psychosis from severe depression but I didn't fit into any of those categories either. The hallucinations caused my anxiety and depression, not the other way around.It was my fourth psychiatrist who knew about hppd and mentioned it to me. I discounted it, forgetting I had once smoked weed that had been laced with what I think was PCP. I started to research hppd and I was amazed that for the first time people were mentioning what was happening to me. I am the one who researched all the the treatments and brought the info to my psychiatrist. I am still going to a neurologist for trigeminal neuralgia but he has no knowledge of how to help hppd.
It is important to rule out other possibilities and have all the tests done.