This he saith, not because they were so, or he thought them to be so, but that he might search out the truth of their affair, speaking too much like a courtier or politician. The nakedness of the land, i.e. the weak parts of it, and where it may be best assaulted or surprised.
Related Commentaries of Genesis 42:9
Genesis 42:9
9 And Joseph remembered the dreams which he dreamed of them, and said unto them, Ye are spies; to see the nakedness of the land ye are come.