Long story short, I'm trying to go through the Volkihar questline without actually becoming a vampire for reasons. I want to be able to trick Harkon and the lads into thinking I'm one of them, but I have no idea how that's done. Initially, I'd think it's a faction thing, given the prevalence of "PCVampireFaction" and "PlayerVampireLordFaction", but it seems more like those are used for tracking NPC hostility. There's also a Global Variable, PlayerIsVampire, in what appears to be the quest that handles vampirism... It could also be just a stage in a quest, but I can't find one anywhere.

Does anyone know how they know?

Or, more specifically, where is the most logical place for the instance where Harkon's like, "You cured yourself, you moron. Here, I'll fix it" Because that seems like the place in which that would be checked... It's a bit of a goose chase.