I think that Slim's observing Lennie's reaction to the pup helps him conclude that he is "ain't mean." Slim observes that Lennie is more enamored with animals, such as the puppies, than anything else. Lennie is not driven by material wealth or social advancement. Rather, he is loyal to a fault, something that is brought out by Slim's conversation with George. Slim is able to match up the characterization of Lennie that he gets through the conversations with Lennie's reaction to the dogs and how much he is enamored with the company of animals. It is through both seeing Lennie and hearing about him through George that Slim is able to make his judgment of Lennie. Lennie's demeanor is one in which he shows himself to be not deliberately mean or wicked. He is child- like, something that is vastly different from the other people on the ranch and something that makes him distinctive from all others. It is in this that Slim is able to understand better than most how Lennie behaves and how his character is in contrast to all others.