In general, they take your word for it, but if they have any reason to believe you might be, they can always try to verify it by calling the organizations you say you volunteered for. Alternatively, they can also ask you about your experiences doing X in an interview, and they can usually smell BS answers.
In summary, don't lie about anything on your app; if you lie and it works, it might help you a little bit, but if it comes out, you're screwed.