Originally Posted by
Kareeah_Indaga
Durnehviir seems to notice on some level (you can call him out on giving you a Dragon name without getting to the point where you yourself find out you’re Dragonborn and his response is "Forgive me, my instinct was to grant you this title. I am uncertain why. Perhaps one day it will become clear to both of us.") and Miraak definitely notices ("What? Who are you to dare set foot here? Ahh... You are Dragonborn. I can feel it. And yet...")
My head-canon on the subject is that your soul gets more noticeable the more Dragon souls you absorb.