How do you know if it's psychosomatic or not?
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