Does Flickr "Know" if a Hotlinked Image Does Not Link Back to Source?

Question

From Flickr's community guidelines:

"Do link back to Flickr when you post your photos elsewhere. The Flickr service makes it possible to post images hosted on Flickr to outside web sites. However, pages on other web sites that display images hosted on flickr.com must provide a link from each photo back to its photo page on Flickr."

Our company currently allows image hotlinking for user-uploaded images. It turns out that this has been more popular than we had expected, and I would like to capitalize on this if possible.

We will be altering the guidelines to include a clause similar to Flickr's, quoted above.

As hotlinking costs us, both in terms of server load and bandwidth, we would like to get at least something out of it, other than merely a warm feeling inside.

My question:

Does Flickr "know" if a hotlinked image does not link back to its source?

Bonus: if Flickr knows, how?

Answer 1

You can log the Refferers on image requests and then simply fetch the page and see if you can find your domain name outside a IMG tag (or simply a link to the desired website). So it's possible, I have no idea if Flickr does this though.