The description of their seizures and the medical history of the patient are used to determine that epilepsy is a possibility. There is no test that is always abnormal in determining seizures, but there are several tests that look at pictures of the brain and can be helpful.
Computerized tomography (CT) is of limited use as it is not very sensitive in finding abnormalities. CT is often done to exclude significant problems first, but magnetic resonance imaging (MRI) sees the brain much better. Small tumors, abnormalities in the brain's shape or blood vessels, and scars from head injury are seen much better and make MRI mandatory where a seizure could be caused by these conditions.
Electroencephalography (EEG), or brainwave recordings, tell us how the brain is working. There are normal EEG waves and others that suggest injury or the risk for seizures. There are different EEG abnormalities for different types of seizures.
The problem with testing is that the majority of MRIs and EEGs are normal in people with seizures and epilepsy, so having normal test results is likely. Normal results can be upsetting, as many patients then want to know how we can be sure of the diagnosis. Normal results are positive, as the chance that the seizures are less troublesome and medications can be discontinued in the future is better. More testing will be done if the seizures are hard to stop but initially these might not be helpful.