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.

This question is on the forums. On there they say that you can add the game to your cart. Send to a friend. and look on your friends list. If they have it. They will not show up in the gifting list!

Hope this helps!