If the agency can find your song and tell you if it's copyright, then good.

If the agency can't find it, then you are safe if the song was published before 1923.

Re: Nelly. Black people in the south before Emancipation could be slaves or free. Unless Stephen Foster mentioned something about it in the lyrics, we will never know whether Nelly and her lover were slaves or free.

However, I once read a historical novel set in New Orleans, and the author had written "She wore the red kerchief which indicated that she was a free woman." Does Nelly wear a red kerchief, perhaps? (Aunt Jemima does.)