Winners are required to allow you to add them on Steam to confirm their DLC ownership (doable by accessing the DLC's store page after the individual is on your friend list) if you request it, else it's grounds for a reroll (same goes for them having to confirm their lack of base game ownership, for private profiles).

That said, if they're an intentional bad agent, they'll probably hide the DLCs (and possibly also base game) they own through temporary deletion, so if you want to ensure proper activation you'd have to deal with the hassle of giving them the key, demanding they activate it right away, and then quickly refreshing all the DLC "friends that own" pages at once to confirm they were all activated at once, to confirm the winner didn't exploit you.

So, yeah.. it's a.. bit.. of a mess. :S

On the other side of things, if you think an individual may have already owned the base game and then hidden it (or have a private profile and refuse to unmask it. or if you just don't want to stress over requesting a private profile winner to reveal their library), SG staff CAN access previous sync records to confirm whether or not that was the case (DLCs unfortunately not being able to be pulled up this way, since that info can't be synced), so you do at least have a more straightforward approach method there.

I think I wont have a problem, I have a GTX 1060, but thanks for the heads-up! :)