At what point in the story did you begin to suspect the true identity of Charles in "Charles"?
The reader should begin to suspect the identity of Charles when Laurie misbehaves regularly, especially when his mother does not attend the Parent-Teachers meeting.
Charles is the name of the the “bad boy” in the kindergarten. Laurie, however, is not well-behaved. We see this clearly at lunch on the first day.
At lunch he spoke insolently to his father, spilled his baby sister’s milk, and remarked that his teacher said we were not to take the name of the Lord in vain.
It certainly sounds like Laurie is a behavior problem. His parents observe this, but also do not expect him to have trouble minding in school. They are obviously naïve and in denial. When Laurie says the teacher spanked a boy for “being fresh,” his mother asks him who and he has to think about it.
Laurie thought. “It was Charles,” he said. “He was fresh. The teacher spanked him and made him stand in the corner. He was awfully fresh.”
This is an early indication that Laurie is lying. As Laurie’s misbehavior continues, we have other hints. His mother does not attend the Parent-Teachers meeting. If she had, she would have found out about Charles earlier. It is not until later, after weeks of Laurie’s obnoxiousness, that she finally finds out the truth—that there is no Charles. The misbehavior her son reports is his.