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

Tom can't hide his excursion because Sid happily points out the changes in the thread on Tom's shirt collar. The original white thread Aunt Polly used to sew on the collar has changed to black. Because of this, Aunt Polly knows that Tom took off his collar before he went swimming.

“Bother! Well, go ’long with you. I’d made sure you’d played hookey and been a-swimming. But I forgive ye, Tom. I reckon you’re a kind of a singed cat, as the saying is—better’n you look. This time.”

She was half sorry her sagacity had miscarried, and half glad that Tom had stumbled into obedient conduct for once.

But Sidney said:

“Well, now, if I didn’t think you sewed his collar with white thread, but it’s black.”

“Why, I did sew it with white! Tom!”

But Tom did not wait for the rest. As he went out at the door he said:

“Siddy, I’ll lick you for that.”

In a safe place Tom examined two large needles which were thrust into the lapels of his jacket, and had thread bound about them—one needle carried white thread and the other black. He said:

“She’d never noticed if it hadn’t been for Sid. Confound it! sometimes she sews it with white, and sometimes she sews it with black. I wish to gee-miny she’d stick to one or t’other—I can’t keep the run of ’em. But I bet you I’ll lam Sid for that. I’ll learn him!”