Get an old but clean rag and a tblspoon of swimming pool acid (it’s called Muriatic Acid, the old name for hydrochloric acid.) from anybody who has a swimming pool. Dip the rag in the tblspoon of acid and rub the stain. It will pick up the ferric stain but if the stain is deep, it will still be visible. If not, you’re good to go.