When they had come to the land of Zuph, Saul said to his servant who was with him, Come, and let us return, lest my father leave off caring for the donkeys, and be anxious for us.
Evidently, the servant had already encountered some of the citizens of that town (very probably Ramah), who had told him of Samuel's recent arrival in Ramah.  He could hardly have known of `a man of God's' being there unless that had indeed happened.  Of course, Saul did not know that, so his servant told him.  It is amazing how often writers forget the abbreviated nature of such narratives as this.