Remember that to the supernatural world and beings of the Nevernever, what one IS is a very simple and obvious thing. It is part of their very nature. I like the example of Toot Toot given above. When Harry went over, so did his "Army" of little Fae. And he didn't even have to send out a memo or anything. The natural order shifted and the new reality took hold.
The White Council claims all mortal casters by default. This was done for two reasons. One, the other, non-mortal signatories of the Accords probably did even consider it could be otherwise. It would be like Mab having to go through and assert that any member of her Court were actually members of her Court... individually. The second reason was that the Wizards are shrewd, paranoid and careful types that understood that, for many reasons, having default claim to all mortal casters was the way to go. Otherwise, it would be open season on young mortals with the gift and any faction could just go forward with a "Final Solution" to the Wizards all nice and legal like. This is why the Red Court had to go to war to do just that.
Now, mortals are cheeky things. They have a free will that is recognized by (if not entirely understood) by supernatural beings like the Fae. So, a mortal can choose to serve or align with another power. But that choice leaves an indelible mark upon their soul that is, usually, readily apparent to anyone in the know. If a new Winter Knight walking around bumped into the Erlking on the street, the Erlking would immediately recognize him for what he is. The touch of Winter would permeate his very being and soul. Same if mortal picks up a Coin, sells his soul to a Dark Power or demon, or swears faith to a god like Odin.
Now, other mortals may not be able to instinctual know. But, to those beings of the Nevernever, it is as simple as reading a name badge at a convention.