Diagnosis of non-specific back pain is done in stages. Your doctor will ask questions and take a history and then will examine you. To start with they will be making sure they are ruling out serious or worrying problems.
This is a process of elimination and when all these other causes are thrown out you are left with simple back pain.
Once you have the diagnosis finding the exact tissue that is the source of your pain is much harder. X-rays and MRI scans only tell half the story and you may find that a very precise diagnosis is not possible. Find out more about how you diagnose simple back pain here.