Curley's wife notices that Lennie has a bunch of bruises on his face. Being Curley's wife, she has also likely noticed that Curley had some problems with his hand. She likely put two and two together and figured out there was a fight. We know this from these two quotes:
"Where'd you get them bruises on your face?"
She turned to Lennie. "I'm glad you bust up Curley a little bit. He got it comin' to him. Sometimes I'd like to bust him myself."
She used this situation with Lennie and the other guys to show Lennie a little bit of appreciation. Even though the guys try to tell her a story about Curley getting hurt in a machine, this added detail with the bruises on his face prove to her that the two men were obviously in some kind of tangle.