A definitive diagnosis is based on a combination of clinical suspicion and the discovery of evidence. Because of the highly variable nature of presentations, there are no specific general diagnostic strategies across all cases. Suspicion may be aroused when the history is either inconsistent with typical syndromes or is inconsistent over time. Suspicion may also be raised by laboratory results that are outside the range expected for a given clinical presentation (e.g., unusual bacterial speciation or elevated serum insulin levels). Sometimes the first clue may be a provider discovering medications or medical paraphernalia in an unusual location in a patient's room. This would be an accidental discovery. In the scenario where the medical team suspected the diagnosis and wished to make a formal search of the patient's room, then legal counsel should be sought.
The definitive diagnosis of a factitious disorder must be confirmed with evidence. There may be laboratory evidence supporting feigned illness or eyewitness evidence of the patient manipulating findings. Once suspicion is raised, further evidence may be gleaned from the medical record, laboratory reports, or, less likely, from the patient interview.
Factitious disorders exist on a continuum of severity, from fairly minor to severe and life-threatening. At the minor end of the spectrum, a patient might fabricate aspects of a history to seem more unusual or exciting. A patient might go further and produce false or misleading findings on a clinical test (e.g., putting a droplet of blood in a urine specimen to simulate hematuria). Somewhat more deceptively, a patient might induce actual physical exam or laboratory findings by, for example, using anticholinergic eye drops to simulate a third cranial nerve palsy or taking insulin with the intent of producing hypoglycemia. Factitious illness is more severe when a patient induces actual disease (e.g., by intentionally contaminating a wound with saliva to induce infection, or injecting stool into the bloodstream to induce fever or sepsis).
Case reports demonstrate the breadth of symptoms, including:
Hypoglycemic seizure in a nondiabetic woman induced by self-administering insulin 
Feigned Usher syndrome (a genetic disorder of deafness and blindness) 
Self-induced chronic infection of a wound site in a living liver donor 
Feigned reflex sympathetic dystrophy (by using tourniquets to induce lymphedema) 
Feigned intestinal obstruction (using syrup of ipecac) 
Self-induced diarrhea (using laxatives) 
Simulated proteinuria (by injecting egg into the bladder). 
Subtypes of factitious disorder include those with predominantly psychological, predominantly physical, or mixed signs and symptoms. In the subtype with predominantly psychological signs and symptoms, patients:
Feign complaints of a perceived mental disorder, whether or not they conform to typical clinical syndromes
May claim common symptoms (e.g., depression or suicidal thoughts) or more unusual complaints (e.g., amnesia or other dissociative symptoms)
May exaggerate symptoms when aware of being observed, most commonly seen in the emergency department or psychiatric unit. 
Factitious disorder with predominantly physical signs and symptoms:
This occurs most commonly of all the subtypes.
Patients create signs or symptoms of an apparent general medical condition.
Munchausen syndrome is a subtype of this category.
Munchausen syndrome is an extreme form of factitious disorder. People with Munchausen syndrome may present with dramatic tales of travel and acute illness, often attracting significant attention from hospital staff. As the story unfolds, the patient is often found to have been to many different hospitals under aliases and with multiple complaints, often leaving against medical advice when it becomes clear that the symptoms are fabricated. The patient may have multiple scars from unnecessary laparotomies, perhaps even inducing unintentional complications, such as bowel obstructions.
Factitious disorder with combined psychological and physical signs and symptoms presents with a mix of physical and psychiatric complaints. 
Factitious disorder imposed on another (previously factitious disorder by proxy) describes the clinical scenario wherein the patient feigns signs or symptoms of illness in another person who is under the patient's care, when there are no clear external incentives.  While these are usually cases of child abuse, disabled adults may also be abused.  Case reports include:
Factitious insulinemia leading to pancreatectomy when the mother repeatedly injected her infant with insulin 
Seizures induced in an infant due to amitriptyline given by the mother 
Pediatric apnea caused by a mother suffocating her infant. 
Risk factors known to be associated with factitious disorder should be specifically asked about during the patient history. Evidence to confirm a suspected factitious disorder should then be sought by medical records review, consultation, patient observation, further clinical interview, and laboratory findings.
If a patient has been seeking care for feigned illnesses at multiple hospitals or clinics, gathering records from these multiple presentations may support a diagnosis of factitious disorder. Providers should seek medical records from other locations where the patient has presented to look for a pattern of unexplained illness. Because of the central role of deception in factitious disorder, patients may be reluctant to sign release-of-information forms. The integrated electronic medical records at some larger health systems, for example at the US Veterans Health Administration, may make establishing a pattern of unexplained illness easier than in systems that use paper records.
When clinical suspicion of factitious disorder is high, it is wise to seek consultation from several sources. Psychiatry consultants may be more familiar with evaluating factitious illness and the differential diagnoses that may have similar presentation, such as somatoform disorders and malingering, and may help guide the evaluation. Risk-management and legal-service consultations may help guide practitioners in establishing when it may be appropriate to obtain records against a patient's wishes or to search a patient's room to establish a diagnosis. They may be required when a patient wishes to leave against medical advice or if the provider wishes to terminate services when there is no true illness to treat.
Because patients are motivated to deceive the medical team, patients are not likely to admit that they have been involved in simulating symptoms or inducing disease. For specific presentations in which a patient is feigning signs or self-inducing actual disease, irregular findings on diagnostic tests may be the most solid evidence that a practitioner has that illness is fabricated. Results that may be used as evidence for the diagnosis of factitious disorder include the following: 
Cultures (e.g., of blood or wound swabs) may be polymicrobial or demonstrate atypical organisms when patients inject saliva or feces to cause infections in the blood or wounds.
C-peptide will be low in instances of self-induced hypoglycemia by injecting exogenous insulin. Historically, insulin antibodies were useful when hypoglycemia was induced by pork or beef insulin. These have become less useful now that human insulin is mainly used. There are also cases of spontaneously occurring antibodies, which complicate interpretation of results.
Sulfonylurea (e.g., glyburide) may be present in the urine if the patient ingests oral hypoglycemic agents to induce hypoglycemia. A good assay is required. Factitious meglitinide (repaglinide)-induced hypoglycemia has been reported, in addition to hypoglycemia induced by sulfonylureas.
Urine potassium may be elevated if the patient ingests diuretics to induce electrolyte abnormalities.
Protein may be widely variable in the urine if the patient adds exogenous protein such as egg white to the urine to feign proteinuria.
Serum thyroglobulin will be low when a patient is ingesting thyroid hormone to simulate hyperthyroidism.
In general, when factitious illness is strongly suspected, invasive and expensive diagnostic tests should be avoided unless absolutely necessary either to establish diagnosis or for patient safety. No diagnostic tests are broadly indicated to rule factitious illness in or out, only tests for specific presentations.