[1] Nobility could be restored by the King's letter of rehabilitation (Lettre de réhabilitation).
That was accompanied by various edicts which declared certain elements of the outfit (heraldry, armaments, decorations, etc.)
In addition to real usurpation, inquiries were made into hereditary nobility.
Unless the nobility was given by a Letter of Ennoblement, according to Grande enquête one had to prove that at least four generations of his ancestors enjoyed the noble status.
[2][3] In Poland, the practice of nagana szlachectwa was questioning the nobility by any accuser.