The biggest clue that Laurie is in fact Charles is how Laurie seems to change so quickly from his mother's "sweet-voiced nursery-school tot" to "a long-trousered, swaggering character who forgot to stop at the corner and wave good-bye to me."
It is this swaggering character that they often witness at their dinner table. He spills his sister's milk, talks insolently to his father, uses bad grammar and gets up from the dinner table in the middle of a conversation without excusing himself. From the beginning, his behaviour and attitude is similar to the characteristics he describes in Charles.
There are less obvious clues in the text as well. For example, at the beginning of the story, he initially finds it difficult to look his parents in the eye when talking about Charles's behaviour, suggesting, certainly in retrospect, that he is trying to hide something. He then has to pause to think when they ask for Charles's name, suggesting that he is making the name up.
The final clue is how much a...
(The entire section contains 4 answers and 1000 words.)