How you you spot a diagnosis that is fraudulent or incorrect?

Get another opinion! This is a bizarre question. Why would anyone make a "fraudulent diagnosis"? One intended to deceive? For what purpose? Can a diagnosis be incorrect? Of course! Health care providers aren't god. Get another opinion, or two, or three, and stop being so suspicious!
Difficult. I know of no board certified physician of any specialty that would ever give a fraudulant diagnosis. Mistake and wrong diagnosises get made every day but in most cases the right diagnosis is made in an attempt to get the patient feeling better. If you have questions and are unsure or uneasy with the diagnosis ask the physicain who made the diagnosis and/or get a second opinion from another doctor.