The answer to this can be found in Chapter 4 of the book. Rufus realized that Larry Dunn had stolen Kenny’s gloves because of what Larry did to disguise them. At first, Kenny did not think that the gloves were his because his gloves were brown and Larry’s were black. But then Rufus pointed out the snow to Kenny. The snow that Larry had touched with his new gloves was all black. Larry had taken the gloves and used shoe polish to color them black. The shoe polish had come off on the snow and Rufus had noticed it.