The census records may contain a good clue to when a person obtained citizenship.
The 1880 census was the first to identify the relationship between the household member and the head of house.
The 1910 census lists survivors of Union or Confederate army or naval service.
It's very important to check maps. Boundaries change over time. Be sure the area where you think your ancestors resided is actually the area where they were.
Federal Census records are available to the public 72 years after they are taken.