Originally Posted by DominicFord

There are a few ways. If the "horse is already out of the stable," you can issue a court order to the filehosts asking for the user's information.

Or, you can watermark the files when they are downloaded from your servers. Then, it is fairly trivial to see who uploaded the file, because when you run the file through the watermark detector it will tell you which of your users uploaded it.

We (PornGuardian.com) offer such a watermarking system. You can email me at [email protected] if you are interested in more information.

Best,
Dominic