It’s extremely important to observe people’s reactions when you’re communicating with them. Why? Well, a significant percentage of communication is actually body language, so scrutinizing how individuals physically react to you when engaging with them will give you important clues.
Verbal and nonverbal activity is a unified whole, and theory and methodology should be organised or created to treat it as such. Kenneth L. Pike
Look at people's eye contact, their posture and facial expressions. If they hold your gaze, turn towards you and even mirror your own actions, then it’s a good sign.
However, a closed stance, or looking anywhere else other than directly at you, means you may not yet have them on side.