If a person is “compatible with magic and be capable of serving in a vital role as a Magical Guardian.” A familiar will be created.
Said familiar will then “gather more information and perform a final assessment”. Known sources of info being all possible info that could be found on an electronic device. as well as by physically following the potential MG for a time, and using some unspecified magic. (Selene followed Mai for 23 days before the emergency contract so we don’t have a good idea the info gathering stage normally lasts.)
Of note the only place that isn’t monitored is inside of the potential MG’s home, hough any online activity still is recorded.
Known criteria:
Being compatible with magic. (Mai had a “mana resonance of 124% when first contracted)
Being willing to fight the Anathema and would use their powers to do so.
Not being the type of person to abuse their power.