Weakness and pain. Usually a herniated disk causes pressure to be put on nerves of that area, causing pain. Cervical disk problems can be localized by a patterrn of motor weakness, and areflexia. This can be seen at the shoulder(c5), biceps(c6), triceps/finger and wrist extensors(c7), finger flexors(c8). Mri is great to comfirm the diagnosis, but it is expensive. See a good neuro. Doctor first.
Myelogram. Of the c spine can also make diagnosis if you can't have mri.
MRI is best. Mri is the most accurate way to diagnosis disc herniation in the spine. Next accurate would be a ct scan myelogram which is a ct after a spinal puncture with injection of ct dye into the spinal sac. After that, a ct scan without myelogram. Regular xrays are useless as the discs are invisible on xray. Mri can be performed without dye injection and doesn't involve radiation, hence its popularity.