There is no surefire way to know for sure, but one of the hallmark signs that a girl is secretly attracted to you is if she breaks the intensity of her stare when she's looking at you. This could be because she realizes she's displaying one of the subconscious signs of attraction and doesn't want anyone to know about it. It could also be because she's not used to dealing with feelings of attraction towards someone, and so breaking the gaze allows her to deal with the feeling in a less public setting.