Summarize this article:

Properly reading body language is vital to asking a girl out, whether she's a stranger or a friend. This will help you to gauge whether she's interested before you even start talking. While researchers disagree about exactly how much body language plays a role in human communication, most agree that it is significant. Keep your eyes on her face and see if she returns her gaze. If she won't look you in the eye for very long or is constantly looking elsewhere, there is a good chance that she is not interested in a date. Remember that there are a few reasons why she may not keep eye contact even if she's romantically interested in you. She may have vision problems, social anxiety, autism spectrum disorder, or some other condition that may make nonverbal communication difficult. If she is interested in you, she will likely face in your direction while keeping her arms open and unfolded.  Signs that she's disinterested include crossed arms and a hunched posture.  Keep context in mind. If it's a cold day, don't assume that a girl crossing her arms is communicating disinterest. Likewise, if you're standing in the only exit of a room, don't mistake her facing in that direction as a sign of interest in you.

Summary:
Pay at least as much attention to what she's doing as what she's saying. Look at her eyes. Check her posture.