As Target was wrote, give me the polygons and I can do it. You can get the polygons from OSM. The polygons will be complex (many points), so the definitions will be long, but this is not problem. The speed is not problem as I can check every found against the only polygon of respective county, so for someone with 10k founds I should do only 10k tests.
However, as SeekerSupreme wrote, the main problem is, that the boundaries are not constant in time, so someone should periodically update the polygons.