Hate to be the new guy here... but-

You can send the key via email. When the winner is chosen, their email will be visible for you.

You can also wait 7 days and if you can't contact the winner you can ask to have the giveaway re-rolled. That way you know your key won't be used.

well, after 7 days, if they still don't respond to your friend request, just choose a new winner

If you are giving out keys then make sure to add the winner and then give him the key directly through steam chat, or else using the email method might be risky. And if the winner of your giveaway doesn't accept your friend request / the key within 7 days, then you can reroll the giveaway and you'll get a new winner.

You can't ask for a reroll as you allready game the key in an email, so you can't know if the key will be unused...