Gestures are the most direct body signals given while interacting with someone. These include the movement of the head (a tilt or nod), any noticeable shoulder movement (a shrug or lean in), hand movements, placing of feet. Note if there are any common gestures that the person tends to demonstrate. Look past the obvious! Notice their posture, is one leg always crossed over another, do they tend to look a certain direction more, etc. All of this can help you develop a baseline and, more importantly, identify when they stray from that baseline.