It sounds like you are talking about when the Murrys first got to Camazotz. Everyone looked very robotic, and the children were outside bouncing balls and jumping rope in unison. All of them appeared to be robotic, but then they noticed one boy who was NOT in sync with the rest. Mothers appear, in unison, and retrieve their children, but the one boy continues to bounce his ball in a pattern that does not match what the other kids were doing. His mother sees this, grabs him and takes him into their house. She is obviously very upset, and the ball is left outside. They take it to the door, the mother denies her child could have done anything not in sync, but the boy grabs the ball. After seeing more instances of behavior that looks robotic (the paperboy's perfect paper throwing, for example), Charles uses his ability to feel what others are feeling, and can tell they are people, not robots.