I have a dog who has epilepsy. We immediately contacted our vet when her seizures started, and he ran several blood tests to determine whether it was from other causes. It's kind of a process of elimination to determine if it is epilepsy or not. There are a couple of websites that have a lot of helpful information you may want to check out:
http://www.canine-epilepsy.com
http://www.canine-epilepsy.net