How does Charles Wallace know that the robotic inhabitants of camazotz are not actually robots?

Charles Wallace realizes that the inhabitants of Camazotz are not true robots because they exhibit behaviors and emotions that suggest they are controlled rather than autonomous. He perceives their lack of individuality and free will, as they all conform to the same rhythm and actions dictated by the central intelligence known as IT. This understanding highlights the distinction between mechanical beings and those who possess genuine thoughts and feelings.