If you have tenderness at the bony part behind your ears, that would be a symptoms of a neck injury. Also, palpating it can help. Plus, as you said, success of treatment is a good sign.

Much of medicine is 'success of treatment' oriented.

An upper neck injury does not rule out a brain injury too. It is best to try to treat both. There is not a real treatment for the brain injury where there is a positive treatment for the neck.

You may have to ask for a referral to find a good neck specialist or even do your own research. Ask about specialists who successfully treat whiplash.