When David is told that he will succeed in pursuing the raiders and in overtaking them, he does not know who they are nor where they have gone. It may be that the LORD provided him with specific directions through Abiathar, but most likely he took his men in the direction that he expected the raiders to flee. There may also have been some signs or clues to indicate the path on which they were travelling.
9 So David set out, and the six hundred men who were with him, and they came to the brook Besor, where those who were left behind stayed.