How does the author show what Billy is thinking in this story?

An interesting point about this story is that there are few insights into Billy's thoughts and that it's only his actions that give a clue as to his state of mind and what he's thinking. For example, he travels around the United States for about a year, working for ranchers. During that time the reader has no idea what Billy is thinking and whether he misses Boyd. Then Billy stops and visits briefly with Mr. Sanders and heads into Mexico. This seems to be an indication that Boyd's absence is weighing on Billy, though Billy never says so.