@Exkalamity Honestly, the biggest giveaway for me is that, in Volume 1, the chapter in which Sue is introduced goes out of its way to remind us that the country in which Arcus lives is a kingdom. Even though the general societal structure and the fact that Arcus lives in a kingdom is already very well-known and clear to us by this point, the author took the time to remind us of it... only for it to have no (obvious) payoff in the context of Arcus's jaunt outside that ultimately led to him meeting Sue (and Cazzy). The rest of the story is well-written enough that it doesn't seem like the author just soapboxing about the world, and it's (IMO) too subtle to be a red herring.
There are other minor things as well, like Sue promising to fix up Arcus's arm as though it's her responsibility to do so (which makes perfect sense when considering that Arcus ruined his arm protecting Ceylan), or the prince and Sue having very similar personalities when it comes to learning about new magics (Sue's reaction to Dwarf Star/Scrapped Arms compared to Ceylan's reaction to Magnetic Butterfly/Volley/Zarach Ohr), or the fact that Ceylan knew about Cazzy's kidnapping background and responded in a flustered way when asked how/why they knew about that.
But ultimately, it's not conclusively proven.