I'm not totally convinced it is always easy to spot. Is there an objective medical test like they have for high blood pressure, high cholesterol, diabetes, broken arm, etc.?
Although mental illness is somewhat fashionable currently, who wants to be tagged as "mentally ill"?
This is what they use to determine depression. DSM-IV
https://en.wikipedia.org/wiki/Major_depressive_disorder#DSM-IV-TR_and_ICD-10_criteria
[h=3]DSM-IV-TR and ICD-10 criteria[/h] The most widely used criteria for diagnosing depressive conditions are found in the
American Psychiatric Association's revised fourth edition of the
Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR), and the
World Health Organization's
International Statistical Classification of Diseases and Related Health Problems (ICD-10), which uses the name
depressive episode for a single episode and
recurrent depressive disorder for repeated episodes.[SUP]
[127][/SUP] The latter system is typically used in European countries, while the former is used in the US and many other non-European nations,[SUP]
[128][/SUP] and the authors of both have worked towards conforming one with the other.[SUP]
[129][/SUP]
Both DSM-IV-TR and ICD-10 mark out typical (main) depressive symptoms.[SUP]
[130][/SUP] ICD-10 defines three typical depressive symptoms (depressed mood, anhedonia, and reduced energy), two of which should be present to determine depressive disorder diagnosis.[SUP]
[131][/SUP][SUP]
[132][/SUP] According to DSM-IV-TR, there are two main depressive symptoms—depressed mood and anhedonia. At least one of these must be present to make a diagnosis of major depressive episode.[SUP]
[133][/SUP]
Major depressive disorder is classified as a mood disorder in DSM-IV-TR.[SUP]
[134][/SUP] The diagnosis hinges on the presence of single or recurrent
major depressive episodes.[SUP]
[8][/SUP] Further qualifiers are used to classify both the episode itself and the course of the disorder. The category
Depressive Disorder Not Otherwise Specified is diagnosed if the depressive episode's manifestation does not meet the criteria for a major depressive episode. The
ICD-10 system does not use the term
major depressive disorder but lists very similar criteria for the diagnosis of a depressive episode (mild, moderate or severe); the term
recurrent may be added if there have been multiple episodes without mania.[SUP]
[127][/SUP]