With this World of Warcraft addon users can check if they have completed a particular quest based on the official Quest ID.
This addon was created so that, when I find quests on Wowhead that are of interest to me, I could check in-game to see if I had completed it without having to edit/copy/paste the traditional macro.
/script print(IsQuestFlaggedCompleted(#####))
In fact this addon simply creates a UI to simplify the process of using that macro.