After a seizure, some people will still have abnormal electrical activity in the brain for hours. Many won't. So you can't prove someone hasn't had a seizure that way. But you can find evidence to support the claim that they have had one, if you're lucky.
Tests will show whether you are susceptible to some forms of epilepsy - not just photosensitive. Epilepsy triggered by lights is rare but it is easy to diagnose.
A head injury, irregularities on a brain scan, a brain tumour could cause epilepsy, there would be supporting evidence if one of these causes was present.
If this was a first seizure and there is a diagnosis of epilepsy, this woman has had at least one seizure since.
While it's prohibitively expensive for public cases where epilepsy can be assumed and medication tested, people can be admitted to hospital, hooked up to EEGs and observed on camera.
A wealthy person may have been able to pay for this treatment, and I'm sure they would after an accident like this. But they would not be able to pay for a diagnosis without some of the positive signs above.
Eye-witness reports, dashcam footage, CCTV and the driver's state and behaviour afterwards could all be additional evidence. For example, not all epileptic seizures cause you to bite your tongue in a specific way, but it's rare to do so outside epileptic seizures.
tldr: there are several ways to show that someone has had a seizure, but they won't work for all seizures. Catching someone in the act of having a second seizure, or provoking one with medical tests, would help to give a diagnosis of epilepsy, and certain abnormalities or injuries to the brain can make epilepsy more likely.