Thank you for the opportunity to help you with your question!
A mental health professional know when to diagnose symptoms
as a disorder after an observation of the associated symptoms. Symptoms are
used as clues that help determine the likelihood of a diagnosis with the
presence of an illness. They are also used to compose a listing of probable diagnosis.
These are called the differential diagnosis which is the basis from which an
initial test is ordered to choose the diagnosis option and choose treatment. With
this evidence of the symptoms, which predict an availability of a disease, the
professor therefore gets to know when to diagnose the symptom as a disorder rather
than figuring them out as normal personality traits.
Please let me know if you need any clarification. I'm always happy to answer your questions.