"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.