The necklace just gave him a random human appearance. They wouldn't be able to identify him anyway because he didn't get this necklace until after the demons lost.

As for identifying Astiroth, according to the intelligence of your character that you have stated, you kinda put it out thhat all it would take is a quick glance.

By my cs, there is only one character who would actually have the ability to detect Astiroth and that's Alionna.  Sue is a holy class worshipping a holy figure that takes care in hunting demons.