Most doctors like to rule out other diseases before diagnosing Fibromyalgia. They will do different diagnostic tests depending on the symptoms.
A lot of patients complain of pain throughout their body, weakness, difficulty walking, balance problems, memory loss, foot problems and skin problems to name a few. Also IBS and severe fatigue.
Some tests may include MRI's and emg, bloodwork ,physical touch exam
The emg will see the electrical conductivity of the skeletal muscle
And the MRI might look for demyelination of the cord, which would help to diagnose Multiple Sclerosis.
The MRI will look for whit areas in the brain and other clues for MS
If a dr doesnt do any test,and just says you have fibromyalgia without doing anything I would Question his validity
An experienced dr might be able to exam you and tell by touching the areas that hurt and have a good idea from what u tell them, but they all should rule out the other diseases.