Miss Strangeworth's vicious campaign of poison-pen letter writing is discovered quite by accident. One day, the repugnant little old lady is at the post office, sending off three more of her venomous missives. Overhearing a conversation by a couple of townsfolk—"her people" in "her town" as she calls them—she immediately sighs, reflecting on the prevalence of evil, even in a charming little town such as this one.
This is a wonderful example of dramatic irony, which is where we know something that one of the characters in the story doesn't. In this case, we know, even if Miss Strangeworth is blissfully unaware of it, that she's the main source of evil in the town, due to her vicious letter-writing activities.
As Miss Strangeworth posts her latest batch of hate mail, two of the three letters go straight into the mailbox. But, unbeknownst to her, the third catches on the side before falling to the ground at her feet. Miss Strangeworth is blissfully unaware of this as she turns to go home to her quiet bed in her lovely house. The Harris boy picks up the letter and hollers after her, but Miss Strangeworth can't hear him. The letter is addressed to Don Crane, and young Harris takes it over to him. It's then that the identity of the poison-pen letter writer is finally revealed.