There is no way to tell how much pain a person has. No test can measure the intensity of pain, no imaging device can show pain, and no instrument can locate pain precisely. Sometimes, as in cases of headaches, physicians find that the best aid to diagnosis is patients own description of the type, duration, and location of pain. Defining pain as sharp or dull, constant or intermittent, and burning or aching may give the best clues to the cause of pain. These descriptions are part of what is called the pain history, taken by the physician during the preliminary examination of a patient with pain.
Physicians, however, do have a number of technologies that they use to find the cause of pain. Primarily, these technologies include:Electrodiagnostic procedures include electromyography (EMG), nerve conduction studies, and evoked potential (EP) studies. Information from EMG can help physicians tell precisely which muscles or nerves are affected by weakness or pain. Thin needles are inserted in muscles, and a physician can see or listen to electrical signals displayed on an EMG machine. With nerve conduction studies the doctor uses two sets of electrodes (similar to those used during an electrocardiogram) that are placed on the skin over muscles. The first set gives the patient a mild shock that stimulates the nerve that runs to that muscle. The second set is used to make a recording of the nerve's electrical signals, and from this information, the doctor can determine if there is nerve damage. EP tests also involve two sets of electrodes?one set for stimulating a nerve (these electrodes are attached to a limb) and another set on the scalp for recording the speed of nerve signal transmission to the brain. Imaging, especially magnetic resonance imaging (MRI), provides physicians with pictures of the body's structures and tissues. MRI uses magnetic fields and radio waves to differentiate between healthy and diseased tissue. A neurological examination enables a physician to test movement, reflexes, sensation, balance, and coordination. X-rays produce pictures of the body's structures, such as bones and joints.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.