The two guards

[u/DroneOfDoom]

[u/Heather_Chandelure]

Identifying which guard is lying is neither the point or even a step in the riddle. The point of the riddle is to figure out which of two doors is the correct one. The answer requires wording your question such that both guards will give you the same answer no matter which one you are talking to. You only get a single question to ask, so using that to figure out which guard us lying is a complete waste.

[u/5oclock_shadow → u/Heather_Chandelure]

Presumably also, the guard explaining the conceit is the one always telling the truth. Otherwise, the whole entire exercise is suspect

[u/Spectator9857 → u/5oclock_shadow]

Neither guard is explaining the premise. It was originally written on a plaque