Question

How did curleys wife know that lennie was the one who broke curleys hand​

Answer 1

She knew that Curley didn't get his hand caught in a machine. Curley didn't run the machines and he was sulking even more than usual. She knows he was rushed to town in the middle of the night and came back with a giant bandage.

Answer 2

Answer: Curley’s wife knows what he’s like and that he most likely started a fight and ended up hurting his hand through a fight, not a machine.

Explanation: she also notices bruises on Lennies face and once she asks about them, he answers.