It is very easy for someone to lie about depression, anxiety or pain. Indeed, patients in general practice routinely lie about the severity of some of their more frightening symptoms. Patients being paid for symptoms frequently emulate the symptoms of post-traumatic stress disorder. It is quite simple to lie about problems with sleep. It is easy to attribute weight gain to depression.
If we relied solely upon what the patient said, we would be quite vulnerable.
The reasons tests are ordered to clarify what is causing a reported symptom; diagnostic tests are also ordered to demonstrate what is not the cause of the reported symptom_and often to demonstrate that the symptom could not even exist.
With all of the media coverage on depression, phobias, generalized anxiety, and PTSD as well as access to the internet, it is very easy to learn what symptoms you are supposed to have in order to be diagnosed with those disorders.
Falsification of psychological symptoms is a great concern in cases where the patient receives compensation. I have seen patients who have received disability payments for 20 years, and yet upon examination, it is clear that they have no psychological disorder which suggests any degree of impairment.
Thus, two additional things are needed: A meticulous history that looks for other possible causes of the reported symptoms (anything from past abuse to current greed). Secondly, what is also needed is a careful examination of the diagnostic data to determine the inconsistencies and/or the excess to which symptoms are being reported.