There are also feel of magical power to compare, no matter how good the disguised, demons with magical average magical power won't be able to imitate it properly.
Even if they do possesses enough magic power to imitate Bachiko's, as stated in the chapter, Bow Creation is a technique that is notoriously hard for demons to master.
Bachiko's other students or members of her family might be able to create a bow, but they won't have enough magic power to give off the aura of a 13 crown. Conversely, someone like Kiriwo might be able to amass enough magic power to pose as a 13 crown, but they won't have the skills to create a Bow as perfect as Bachiko's.
So really, if there's someone who could pull off such a tremendous skill, with the prowess of a 13 crown, then by all rights that someone HAS to be either Bachiko herself, or a demon equals to/surpasses the 13 crowns.