A diagnosis is made after other possible causes for symptoms have been excluded. This should be a positive clinical diagnosis made on a well-characterised constellation of symptoms.
A doctor will assess medical history, recognise the typical symptom pattern of M.E. and conduct basic tests, for example on blood or urine samples, to rule out other conditions.
Many illnesses can sound quite similar in their symptoms and excluding other conditions is an essential part of the process to inform a diagnosis.
It may take several visits to a doctor before a final diagnosis is made.