How would one come to the diagnosis of a conversion disorder?

Challenging. It is a diagnosis of exclusion. It can be very tough to diagnose - unless you are able to determine the purpose it is serving (which the person would not consciously be aware of)
Conversion hysteria. typically involves the sudden and dramatic onset of a catastrophic physical symptom such as blindness, paralysis, seizures or other symptoms.
Careful evaluation. Challenging to diagnose, conversion disorder is a rare "somatoform" disorder in which a person loses voluntary motor or sensory function, suggesting a neurological or other general medical condition. The onset of these symptoms is preceded by conflicts or other stressors and can't be explained by a medical condition. The person is not "faking" the symptom.