No special requirements for a sign in our state. They can pretty much post any way they want, as long as it is conspicuous.
§ 14‑415.11. Permit to carry concealed handgun; scope of permit.
(a) Any person who has a concealed handgun permit may carry a concealed handgun unless otherwise specifically prohibited by law...
(c) A permit does not authorize a person to carry a concealed handgun in the areas prohibited by G.S. 14‑269.2, 14‑269.3, 14‑269.4, and 14‑277.2, in an area prohibited by rule adopted under G.S. 120‑32.1, in any area prohibited by 18 U.S.C. § 922 or any other federal law, in a law enforcement or correctional facility, in a building housing only State or federal offices, in an office of the State or federal government that is not located in a building exclusively occupied by the State or federal government, a financial institution, or on any other premises, except state‑owned rest areas or state‑owned rest stops along the highways, where notice that carrying a concealed handgun is prohibited by the posting of a conspicuous notice or statement by the person in legal possession or control of the premises...