How can I tell which DLC my friends have for their games?

DLC's have their own store page. If I look at the base game store page for Civ V for example, it shows me this in the right-hand-side:

If I then click (or hover) on the title of a DLC in the DLC list, (e.g. Brave New World) the right-hand-side shows me which people on my friends list actually own that DLC.

If your friend is already in that list, they have that DLC already.