No structural lesion. There is a lowering of the seizure threshold due to an abnormality of ion channels or receptors. Many seizures that manifest clinically as a generalized tonic clonic seizure have a focal onset that is so brief or subtle that it escapes clinical detection. The EEG may show the real problem. (Onset in a location with rapid spread of abnormal electrical activity to the rest of the brain.).
Scans are Limited. Scans about creating an anatomical image. However the spacial resolution of all current scans, at their smallest scale, remains quite large/huge compared to cells. Ex: CT scan resolution down to ~1 mm pixels (i.e.1 mm squares as the smallest image elements within a calculated CT 2 dimensional image) contain millions of cells & don't see activity. Study: http://goo.gl/SsnRVP & http://goo.gl/4YdKsa.
Brain is complicated. The brain is a very complicated organ. Epilepsy is when a person has repeated seizures over time. There can be many different reasons why someone has seizures. Seizures are due to abnormal electrical activity in the brain. Therefore an electroencephalogram (EEG) that detects and records electrical brain activity is use to diagnose seizures/epilepsy.
Seizure workup. Depending on the history and physical and laboratory exam by a Neurologist, EGGs and perhaps brain scanning can be done looking for tumors, A-V malformations, etc. It is up to his judgement not routine.