Here are links to a couple of threads you might want to read that will give you an idea of how MS is diagnosed:
http://www.healingwell.com/community/default.aspx?f=32&m=457357
There is no one test used to determine if you have MS. Instead, a group of tests are done, including MRI's of your brain and spine, sometimes a spinal tap, evoked potentials, and some others. The neuro examines you, puts you through a series of physical tests in his office, checking your balance, sense of space, that sort of thing..
and everything else it might be that could cause your symptoms is ruled out...and then the doctor may diagnose you with MS.
Many of the symptoms of fibromyalgia look like MS from the outside, but there are specific pressure points at your joints that are tender upon touch, which lead to a diagnosis of fibromyalgia, which don't occur generally with MS.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....
Uppity did a good job explaining things. If you have any more questions, don't hestitate to ask.
Kimber