Are mental illnesses only relatable thru’ direct observation, experience and anecdotal evidence?

It’s impossible (or is it possible) to describe the feelings and perceptions to anyone without training, even those suffering can only understand in good moments when they are in a lucid state. As a recognized disease (see DSM-IV) it could be dismissed as psychosomatic, but there is the contradiction, if it’s all in your head then doesn’t that lead back to a mental disorder? I’m ignoring the CT scans and MRI, and chemical imbalances. Fibromyalgia is a condition that is not understood, and someone without it could not see why this person is in pain without cause. Most people have been engrossed in a novel (I’m relating now with simile) isn’t that like a personal reality, even if the book is deep in the human experience, or universal? I can only state that an over active imagination (euphemism for behavioral health) is still part of the experience. Would you give symptoms that would give you a layman’s diagnosis of crazy, insane, mentally ill? However you would label it.