I have the idea in mind that at the moment you complete a quest by getting the reward from your quest giving npc that this cast a questcomplete spell on you that is displayed like maybe a healing or a lvlup effect --> an aura of light for 2-4 secs that is displayed around the char and can be seen by others.
also a special sound will be played, one that is in game but as i might think not in use --> quest_complete_mus_1.mp3