Diagnosing a traumatic brain injury is a multi-step process. It starts with the patient’s condition at the scene and looking at all factors at play. Did the patient lose consciousness, and for how long? Does the person have a memory of the incident or are they amnesiac? Most importantly, imaging studies. In this day and age, we have very good imaging techniques to detect even the smallest bleeds in the brain and the smallest evidence of traumatic brain injury, even years after the fact.
Also consider the testimony of before and after witnesses. Brain injuries affect people in their day to day functions. The best evidence of that is finding the before and after witnesses, the people who knew the person before they were injured and after they were injured, getting testimony from those witnesses about all the little and big way that a brain injury has changed that person’s life and their ability to perform activities in their everyday life.