Nearly everyone whose magic originates from the Demis can sense the magic around them. Most magical individuals don’t care enough about the ability to train it into a useful talent, Bjorn cared enough. Demi based magic signatures have a golden hue; spirit based magic signatures have a purple hue. If Fae didn’t have one of the keys with her, her signature would be prismatic unless she was channeling the abilities of anyone in her immediate vicinity.
I’ve never really talked about the spirits because they’re not very plot relevant until the very end of Eternity Complex. The spirits are mistakenly seen as one of the Demi based races by nearly everyone and are often referred to by those who don’t know as fairies and/or pixies (and are represented as such in art). The spirits are also thought to have died out not long after the Demi’s defeat at the hands of the first Hope but the reality is that they no longer needed as active a presence in the world of mortals once most of the Demis were gone. There’s three main versions of spirits (it is more of a spectrum than set points): revenants, wraiths, and spirits. Revenants exist solely on impulse and intuition; they have little to no control over their abilities. Wraiths have varying degrees of composure and control over themselves and their abilities; however, they are prone to lapses in control depending on their age. Spirits have complete control over themselves; they are also the only group that fully comprehends what it means to be a spirit. Astor and Adrik are both wraiths but Adrik is very close to the revenant side of the spectrum and Astor is nearing the spirit side.