For somebody else? Same as above but few extra steps:

1. Add that person on Steam.
2. Click on the game artwork on top left of giveaway page or in other way open DLC page.
3. Check 'Is this DLC relevant to you?' box. There's "XXX friends already own this game" with option to list all friends having this game/DLC :)

[Edit]
Some DLCs show on profiles (https://steamcommunity.com/id/pAULSG1/games/?tab=all)... but the way above works for all the DLCs.