In response to the second answer, Candy is not Curley's Wife. Curley's Wife does not have a name, and Candy is the old handyman on the ranch.
When Candy visits the farm hands, she clearly reveals her dislike and contempt for them and Curly. She is disparaging when she tells them she knows Curly was injured in a fight and not an accident like he indicated. It is not, however, until she sees the bruises on Lennie that she makes a real connection to what happened to Curly. At this point she sets out to charm Lennie, perhaps because she feels she'll be able to control him or use him in the future as a weapon against Curly. Setting Lennie up for her future use, she continues to flirt and praise him. These actions cause Lennie to see her as a women of interest, and it is these very actions that set up the final tragedy as it plays out in the barn and beyond.
In Chapter 4, the chapter directly after Curley's assault on Lennie, Curley's Wife knows that her husband did not get "his hand caught in a machine." This is the story that Slim forces Curley to tell his dad and anyone else so that Lennie and George don't get in trouble. Curley agrees to this because he's in pain, but also because he doesn't want Slim or the others to tell anyone that he was defeated by Lennie--it would bruise his ego too much.
Curley's Wife brings comments that she thinks Lennie is involved somehow and tries to use this knowledge as a sort of blackmail. She is so desperate for someone to talk to her that she is willing to threaten almost anything to force a conversation (in Chapter 4, she threatens Crooks with a rape charge because he tries to expel her from his room in the barn). Her loneliness and isolation are all encompassing, and her voicing her suspicion is simply a ploy to be included.
Curley's wife suspects that the excuse of Curley getting his hand caught in a cultivator was false, and that she knew that Curley goup is hand bperson because of another reason. Because of this she went searching for the one who was able to injure Curley without worrying about the problems it may cause. When she went to Crooks' room she found Lennie, whos' face was all bruised and scratched up. By finding out about Lennies' injured face she concluded that Lennie must have been the one who was able to injure Curley.