There isn't a true way to test this. You can kind of get an idea from the Volhard testing. But this is just another, of a number of questions, to ask the breeder. What is the pecking order in this litter? You have to remember that dogs are pack animals. And in every pack a pecking order is established. You see this in almost every litter, to a small degree, where there are puppies who try to establish themselves as dominant and some who are more submissive. Dominant puppies almost always try to assert their dominance over the mother, and the mother usually puts said puppy in it's place. That's normal and expected. In fact, this is one of a number of reasons why puppies shouldn't be separated from their litter and the bitch until 8-10 weeks of age. Much of this type of behavior is dealt with and learned during this time. Aggression toward other dogs is quite normal for a pack animal. Dogs don't think people are "part of their pack" like some people would try to lead you to believe, so aggression toward other dogs and not people isn't at all surprising. Outside dogs are seen as just that, outsiders, and a dominant dog will push the subject. So, it's tough to say about your particular dog. How old was he when you brought him home? Was he prematurely separated from it's mother? Was he dominant in the litter? I think one of the best thing anyone can do for a puppy is to socialize it...a lot. Take it to the pet store, dog shows etc. Anywhere where it's a strange place with lots of strange people and creatures.