Most depressed patients will say that _something is wrong_ but will also state _but I do not think I am depressed._ They will go on to describe changes in appetite, significant weight changes, forgetfulness, irritability, problems with concentration, decreased libido, pessimism, self-doubt, early morning awakening, agitation, loss of interest in their hobbies, desire to be alone, impatience, low frustration tolerance and numerous other symptoms that, when combined, indicate clinical depression.
However, if a person has a need to be seen as depressed for some ulterior motive, they can learn to parrot those symptoms on demand. This is where formal, standardized psychodiagnostic assessment is critical. There are checks within these instruments of the validity of complaints made by the individual.
Although it is possible to suspect that an individual is depressed based upon a clinical interview, the psychodiagnostic tests are need to confirm this as a diagnosis.
There are patients who measure as depressed but less depressed than they are focused upon bodily complaints. That is, the diagnostic instruments indicate significantly less depression than the physical concerns would suggest. Something is keeping the patients from being more depressed than we would expect. In those cases, the existence of secondary gain may be in operation, and the patient may be found rewarded for his/her complaints through additional attention, affection and relief of responsibilities.
Thus, you are correct, we cannot solely rely upon a patient_s statement that they are depressed. We need to look for specific symptoms and use standardized instruments to determine the severity and validity of those complaints.