I think that what we humans think makes us different from animals is that we are supposed to be less savage and brutal than animals are.  So I would look for a part in the book where Buck is being savage and brutal.

To me, the best place to see that is where Buck fights and kills Spitz to become the leader of the pack.  This is, as I say, a very brutal scene.  All Buck has in his mind is to kill Spitz.  He wants to kill Spitz because he doesn't like Spitz and because he wants to be the one in charge.  I think most of us would like to think of that as animalistic behavior.