The force is one of many ways he knows. The good he sensed in Anakin, the conflict. That good sense or feeling is Hayden. He can sense it in Hayden it appears. Not to mention with the similar Anakin Epi.3 to Luke Epi.4, he might have seen his picture before. But anyway, it is the force. "other places, the future, the past, old friends long gone." I then think Anakin Skywalker would qualify as a friend or at least something from the past of Luke's.