Sabriel is given one by a Charter mage following her birth and death, although her Father brings her back. Nicholas Sayre also receives one after the Destroyer is taken from his body by the Disreputable Dog, who then sends Nick back into life.
Baptism is the process by which a person is marked as a charter mage, usually at birth. This mark takes the form of an ash-colored charter mark on the forehead of the mage, and gives them the ability to see, sense, and use Charter Magic. Virtually all citizens of the Old Kingdom are baptised at birth, although most never develop their skills beyond a rudimentary level.
A common security practice among Charter Mages is to check the authenticity of a person's Charter Mark for purity. The action required is merely placing a finger, though usually two, on the Charter Mark of the person being checked. The Charter Mage checking the authenticity of the Mark is then able to sense the never ending flow of the Charter, or the mark will decay and reveal either a Mark corrupted by Free Magic or something else entirely. Hedge had a fake Charter Mark at the beginning of Lirael, but it was removed within short order; it revealed an ugly Free Magic-corrupted mark of his own.