In Chapter 4, Lennie and Candy and Crooks are talking in Crooks' room in the bunkhouse while all the rest of the men have gone to town without them. Curley's wife surprises them; she isn't supposed to be at the bunkhouse, but she's bored and wants company. She makes two observations that show that she is smarter than she seems.
The first is that she knows where the other men have gone, although none of the remaining men will tell her. She reasons that "They left all the weak ones here.... Think I don't know where they all went? Even Curley. I know where they all went."
The second major point that shows she isn't as dumb as she seems is that she works out what happened to Curley's hand even though the men lie to her when she asks. After she asks, Candy says, "Why... Curley... he got his han' caught in a machine, ma'am. Bust his han'."
She watched for a moment, and then she laughed. "Baloney! What you think you're sellin' me? Curley started som'pin' he didn' finish. Caught in a machine--baloney! Why, he ain't give nobody the good ol' one-two since he got his han' bust. Who bust him?"
Candy repeated sullenly, "Got it caught in a machine."
Since Curley is always bragging about how he's going to punch everyone he doesn't like, she knows he tried to bully someone.
She changes the subject but before she leaves, she looks at Lennie and says, "Where'd you get them bruises on your face?"
Lennie doesn't know how to respond, so he looks at Candy.
"He got his han' caught in a machine," he said.
Curley's wife laughed. "O.K., Machine. I'll talk to you later. I like machines."
They change the subject again, but before she leaves, she turns to Lennie and says, "I'm glad you bust up Curley a little bit. He got it comin' to him. Sometimes I'd like to bust him myself."
Nobody is willing to tell her anything, but she knows how to read between the lines.