How do you know if it's psychosomatic or not? "It's in your head" is often true and often offensive. But as your head can invent any number of psychological and physiological symptoms how can you distinguish whether something is or isn't psychosomatic?

My naive stance (which I'm looking to correct) so far is that if the symptoms are too vague or too arbitrary it's psychosomatic. This probably has some indirect legitimacy but is entirely anecdotal. For example something too arbitrary would be: experiencing pain in your feet after standing in water in a shower, but standing in a bathtub, pool, lake, or ocean doesn't cause it.
I believe 'psychosomatic' describes a way the mind has effects on the your body which might result in somatic symptoms. Often psychosomatic disorders are diagnosed as such when: 


no somatic correlate to the experienced symptoms can be found
somatic correlates do not sufficiently explain the experienced symptoms


This often results in patients who visit different doctors over a longer period of time, desperately looking for a reason for their symptoms.

Also psychosomatic can refer to diseases which have a clear somatic correlate (e.g. Hashimoto-Thyreoiditis) but the cause of the disease is unknown and the symptoms are influenced by your state of mind (e.g. symptoms worsen under stress). 

However as medical diagnostic tools are limited, it cannot be excluded that there is really no somatic cause for psychosomatic disorders.

ICD-10 diagnostic criteria for psychosomatic disorders are available online, e.g. for somatization disorder:
http://apps.who.int/classifications/icd10/browse/2010/en#/F45.0