Getting a valid diagnosis is very difficult. The Brain Trauma Foundation has been working on diagnostic criteria for concussions. They were awarded a federal contract to set the standards. I have not read anything saying they have completed their work.

A diagnostic that is valid but not accepted by doctors is qEEG using one of the well studied databases. The validity of diagnosing a concussion is 97% or so. Do a Google for "qeeg databases concussion" and you will find some good information.