Wide World of Quests

Unfortunately Blizzard doesn't seem to provide a way to know for sure which world quests you have and have not done, even though they must have this data saved for them to be able to track it. As another poster mentioned, there is an app you can install to track it, but you have to have done so before starting the achievement, which I'm guessing is relevant to very few people who have ended up on this page. Also a "blue" quest does NOT mean you have not done it--it merely indicates the rarity/difficulty level/reward quality of the quest.

Here are some tips from looking around myself:

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2. From my experience, the world quests that have the same name but are in different locations count as different in each location--i.e. each time you do the Tortollan "Beach Head" world quest in a unique location, it counts as different.
   
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4. Each of the pet battle world quests is unique, so if you know you haven't been hitting those up, you can knock out quite a few that way.
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6. Other users report that war campaign/invasion quests do not count at all for this achievement.
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8. Many of the "blue" quests are to kill a particular rare (silver dragon portrait) mob. If you see such a world quest, hit "Y" to open the achievements menu, click "Exploration" then "Battle for Azeroth" and scroll to the "Adventurer of <BfA Zone>" achieves. Alternatively, you can just search for "adventurer of <whatever zone the quest is in> using the search bar. Either way, click on the relevant achievement, and you can see if you've already killed that mob because it will be highlighted in green. If the mob name is in grey, you haven't killed it before so you know you haven't done that world quest yet. If the name is green and you have killed it, it's possible you killed it but not for a world quest, so if it's close by, have at it, but you can use this to narrow down world quests for rare mobs that are definitely worth your time and effort to get to them.
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Hope you found these useful and good luck!

This achievement is account-wide; if you have multiple max-level characters their WQs all count.

I have to say, this undoubtedly has to be one of the dumbest achievements ever put in this game.

The fact that we had to start using an addon BEFORE doing any of the world quests to even be able to track which ones we've done... just... why?

This has got to be some weird attempt at provoking people lol.

I'm currently at 84 or something and 90% of the time, I end up just doing a quest i've already done (obviously)

They should've made this just complete 100 world quests.