Factory OE fenders will have a (matching) VIN sticker.
OE replacement fenders will have a sticker that says R-DOT.
Aftermarket replacement fenders will not have stickers, but have a stamp with their name. Not to insult your intelligence, but I am going to say it anyway, if it says anything other than FORD, it is aftermarket replacement.
You can tell if it has been painted most obviously by color match, if the color match is good, look at the jamb. If it has been painted, there most likely will be a paint edge of some sorts.
Let me know what you find.