Actions speak louder than words, and it is their actions and concern for one another's happiness that speaks of how much they love each other. They each sacrifice their greatest worldly treasures in order to get something for the other person that they think will make them happy. In the entire world, "there were two possessions of the James Dillingham Youngs in which they both took a mighty pride. One was Jim's gold watch that had been his father's and his grandfather's. The other was Della's hair." But, they both are willing to sacrifice those possessions for the other person. This is the point of the story, and the evidence of their love for each other.
On a smaller scale, as Della is thinking about how she wanted to get something nice, she thinks, "Many a happy hour she had spent planning for something nice for him. Something fine and rare and sterling—something just a little bit near to being worthy of the honor of being owned by Jim." This statement shows her love for him; she wanted to get him something nice, because Jim was such an honorable person. Later, when she finds the watch-chain she thinks, "As soon as she saw it she knew that it must be Jim's. It was like him. Quietness and value—the description applied to both," It is obvious from this that she loved and respected him greatly.