How do you know a Humble Bundle url is unused?

When you use the URL, it will ask you to input your email.

Visit the URL; it'll tell you if it is or not. if not, don't give it your address, for obvious reasons.

So if it looks like this when I get to the page, was it used?
http://i42.tinypic.com/2bxnqg.png

It's been a while since I bought a bundle, and my memory is bad. :U

Unused URLs look like http://www.humblebundle.com/?gift=XXXXXXXX
You can visit them and they show a form resembling a receipt slip, asking for your email address.
Once you've entered your email address it sends a new URL to the address you gave it and the gift URL stops working.

Used URLs look like this http://www.humblebundle.com/?key=XXXXXXXX
They have already been tied to someone's email address and are not for gifting.