Abused or neglected dogs can be retrained but with dogs that are naturally aggressive I have never heard of it.
There was a book I had a few years back that had a chart that told you what signs to look for in a puppy that would tell you if he was aggressive or not. It was actually for Dobermans but I am sure there are tests for Shepards as well.
One test that I am aware of is to pick up the puppy and then put him down. Then you walk away from the pup. If the pup follows you, he's going be an easy-to-train pup.