How did Aunt Polly find out that Tom had been out swimming?

When Aunt Polly questions Tom about what he's done that day (she thinks he skipped school to go swimming), Tom gives an excuse for his wet hair, and then points out that the collar of his shirt is still sewn tight....... thus he couldn't have taken it off to go swimming and it would be soaked through if he had. Sid takes this chance to pipe in and remind her that the collar had been sewn with white thread whereas now it was black. Busted........ Tom skipped school, took off his shirt, went swimmimg, and then sewed it back up!