The MRI has been the chosen method for diagnosing and then tracking the progress of MS. Now there may be a less expensive technique used as a marker, and it may be more detailed than the MRI in some instances. This technique may be new for MS, but it is likely you have been tested with it already. I know I have!
Neurologists from the University of Buffalo have found that optical coherence tomography (OCT) is useful in MS, especially optic neuropathy which is the first sign of MS for 20 - 25% of MSers. This is the same test that has been used for glaucoma. This test works because the retina has no myelin.
Read more about applying this procedure to MS here.
Added March 11: I found this interesting story on Being Ammey. Her diagnosis originated in an eye test.