It takes a great deal of careful observation during these moments of play as well as intuitive responses when boundaries are necessary. Magda Gerber suggests that we provide an appropriately structured environment in which all children are free to make their own decisions and discoveries, interfering only when a child's behavior is not constructive. Limitations should be placed on safety - hitting or hurting- or intentional testing of boundaries. Children should be allowed to struggle during their play, but have an attentive adult nearby to support when necessary. This approach will foster a child's ability to concentrate on their tasks for longer period of times, be active problem solvers and be better able to negotiate social encounters as they grow.
Check out this short video on "Baby Games: How Infants Develop Social Skills" to observe a beautiful example of attentive, intentional observation in the earliest social play.