I don't remember exactly how dx occurred - it was a process. X-Rays and MRIs I think. Then a myelogram (dye in the spine) by the surgeon to identify exactly where to operate.
I do remember (before the myelogram) the surgeon having me stand up straight and relaxed, then tapping me on the top of the head and asking what I felt (which I don't remember - sudden increase of left-arm tingling maybe). Whereupon he immediately agreed I would benefit from neck surgery (fusion of C6&C7).
PS: I had been in two car wrecks the same year many years before.