is there a command line, or even a mod that will tell you a certain quest that you have completed?
Yes, if you know the quest ID of the quest you can just do something basic like:
/run print(IsQuestFlaggedCompleted(questID))
There are quite a few mods that can tell you that a quest is completed, and even which ones you should be able to do. I am biased but I would suggest the combination of Wholly and Grail if you are interested in a lot of capabilities like seeing map pins, etc.
And if you aren’t sure of the quest ID, you can look up the quest on Wowhead. The quest ID is in the URL bar, like this:
www.wowhead.com/quest=47254/the-originals
This command is not working anymore?
This thread appears to have an updated macro:
https://us.forums.blizzard.com/en/wow/t/checking-quest-completion-status/745985
Ditto. Why the heck does Blizz have such an issue with us checking if we’ve done a quest???
New macro command is:
/script print(C_QuestLog.IsQuestFlaggedCompleted(43812))
Compliments of: