Dogs can have seizures for a number of different reasons. Idiopathic/Primary Epilepsy is one such reason and unfortunately it does occur in Australian Shepherds.
Most affected dogs will have started to have seizures around 2 years of age. Some can develop the disease at a much younger age, others may not develop it until a later age (even as late as 6 or 7).
There is no definitive test to diagnose epilepsy, and instead it is a case or ruling out all other possibilities. This includes blood tests and often MRI and Spinal Tap. There are medications available to help control Epilepsy however it can not be cured and the dog is still likely to have seizures at intervals, which could be days/weeks or even months, depending on the dogs.
There is considered to be an inherited link to Idiopathic/Primary Epilepsy, however at the moment the mode of inheritance is unknown. All breeding lines will have cases of Epilepsy somewhere in the background. A good breeder will know where all these links are in their pedigrees, and should be doing their best to reduce the chances of Epilepsy occurring. However there are never any guarantees. Breeders should be open about occurrences in their lines.
For much more information and detail on Epilepsy please visit the Australian Shepherd Health and Genetics Institute website