Many nonverbal cues are deviations from normal behavior. In order to judge when someone’s behaving abnormally you first need to an establish a baseline for their standard reactions. This takes acute observation.
As you interact with someone, notice what they do with their hands. Notice their posture, their stance, and where they look when they’re excited or nervous. Look for any tics, feet shuffling, or uncomfortable pauses. Once you know how they react normally you’ll be prepared to detect alterations.