Obtaining quest ID for flagging macro

I've been hunting all over the web for this and I can't seem to figure it out.

IsQuestFlaggedCompleted() requires you to pass it an ID. Many times some kind soul in the comments on Wowhead will provide this, but I cannot for the life of me figure out how they are finding these ID.

The situation:

I'm trying to figure out if I've missed any of the leatherworking treasures. For the base ones, one of those kind souls posted on [this guide](https://www.wowhead.com/guide/professions/knowledge-point-treasure-locations-dragon-isles) all the ID needed.

For example [Decayed Scales](https://www.wowhead.com/object=380593/bag-of-decayed-scales) have an ID of 70294. Running the command with this ID works perfectly. After I picked up the item, the return became true. Everything I've found online says the quest ID in-game is the same as the Wowhead URL. This might be true for literal quests, but this doesn't work for these objects — the Decayed Scales container URL says 380593, which is clearly not the same ID.

Most specifically, I need to check for [Lava-Forged Leatherworker's "Knife"](https://www.wowhead.com/object=398785/lava-forged-leatherworkers-knife) but obviously I can't check for ID 398785.

While a reply that gives me the correct ID would be lovely, what I'm really hoping to learn is how people actually *find* these ID.