Is there a definite test for multiple sclerosis?

No definite test. There is no definite test for MS. MS is not the same for every one, and the tests and tools we use to make the diagnosis can also find other illnesses which cause similar findings.
No. Even mris of the brain are not specific enough. The diagnosis is a compellation of tests including the lumbar pucture, x-rays, and clinical presentation, and thorough exam. Electrodiagnostic studies are also used.
PERHAPS, UPDATE. Mri software advancing, newer techniques get close to full confirmation of clinical diagnosis. Software finds both old and new lesions, dissemination in time; and scattered locations, brain and/or neck, dissemination in space. Double inversion recovery MRI films pick up gray matter lesions, adding to accuracy. Long-term correlations increasing accuracy, and outcome predictions. Stay tuned!