Multiple reasons. Epilepsy, a condition characterized by recurrent, unprovoked epileptic seizures, has multiple causes. These include: genetic, head trauma, certain brain infections, and stroke. Even with the most sophisticated diagnostic techniques, there are a significant number of cases in which the cause is not known. This, of course, does not mean that the person does not have epilepsy.
By EEG. Epilepsy is diagnosed by the description of the seizure by the patient or a witness and an eeg. Seizures can be precipitated by a number of factor including drugs sleep deprivation and very low blood sugar. Excluding these and other possilbe causes is important in diagnosing epilepsy. Depending on you age, imaging of the brain may be indicated.