How does Aunt Polly know that Tom has been swimming instead of being at school?

In Chapter One,  Aunt Polly tries to trick Tom into admitting that he played hooky from school that day to go swimming. But Tom, aware of Aunt Polly's motives, has sewn his shirt collar back in place after his afternoon swim. Aunt Polly apologizes to Tom for her suspicions, until Sidney - notorious for being "the Model Boy of the village" - points out that Tom's shirt is sewn together with black thread instead of the white thread that Aunt Polly had used that morning. Before she can punish him, Tom darts out the door and runs away from the house.