How does Fern prove that she can talk to the animals?

Fern tells her father the status of Uncle Homer's goslings. She also tells him that Templeton received the information from the goose.... Templeton then passed the news along to Fern.

"Did you know that Uncle Homer's goslings had hatched?" asked Fern.
"How many?" asked Mr. Arable.
"Seven," replied Fern. "There were eight eggs but one egg didn't hatch and the goose told Templeton she didn't want it any more, so he took it away.""The goose did what?" asked Mrs. Arable, gazing at her daughter with a queer, worried look.
"Told Templeton she didn't want the egg any more," repeated Fern.