Hard to tell what each DLC bundle contain when reading up on them on the ESO webpage.
Turns into an "image recognition game", but even that can fail. Like with "Season of the Dragon DLC bundle", where the green dragon to the right is not from the image used for Dragonhold on the same page.
Ingame it is easier to tell. As the different icons state what each item in the DLC bundle is. However, they could have been sorted (by release date?) or "marked" in some way so I could easily tell which parts of a bundle I already own.
Tiny selfish gripe at the end; I usually end up buying the world DLC-s in some form. Which means these bundles end up offering two dungeon DLC-s, and not worth it for me. Not sure how common my behaviour is. Sure your sales can tell.
Anyway, turns out that forementioned "Season of the Dragon DLC bundle" actually hits all three missing for me, so you got me there. Just wish it was easier to spot the deal.