How does Mrs Loftus figure out that Huckleberry Finn is not a girl?

HIs actions. He also changes his name on accident. His posture when he sat down was a major giveaway as well.

He first says his name is Sarah Williams. Then later saying his name is Mary Williams.

He also didn't close his legs when he sat down wearing a skirt. And thirdly, he poked the yarn when he was supposed to use the needle and thread.