Because London doesn't go into a great deal of detail in his book, we have to glean what the characters are like by their actions. We know Beauty is a giant jerk because he beats White Fang: the poor dog "was kept chained in a pen at the rear of the fort, and here Beauty Smith teased and irritated and drove him wild with petty torments" (17.1). And we know how nice Scott is because of action-oriented details like this: "The patting movement slowly and carefully changed to a rubbing of the ears about their bases, and the physical pleasure even increased a little" (20.12).
White Fang is sneaky, but he's not really deceitful. He's a wolf after all; they don't lie. So while human characters could lie or deceive with their words, their actions tell us exactly who they are, be they gigantic creeps or angelic heroes.
When London wants us to know something about a character, he comes right out and says it. There's no way to misinterpret "Beauty Smith was a monstrosity" (16.7) or "White Fang became a fiend." (17.1). Once we have the info, he moves right along, trusting that we don't need anything more to figure his characters out.
London doesn't result to physical descriptions often. That's just not how he rolls. At a couple of key points, though, he dives full-bore into what people look like. The most notable one is his treatment of Beauty Smith, who gets a whole section of description all to himself, which begins with the hilariously on point "He was preeminently unbeautiful" (16.4). It lets us know right away that he's bad news, and all the physical description that follows only confirms our suspicions. Dude's ugly, inside and out.
He does something similar (if more briefly) for White Fang, covering his ultimate kickbutt qualities with a few quick words: "Fully five feet in length, and standing two and one-half feet at the shoulder, he far outweighed a wolf of corresponding size" (17.3). These descriptions are more notable for their rarity—we don't get passages like this very often in the book. But they definitely leave an impression, and true to London's dedication to not saying anything more than he has to, they get right to the point.