Early in the book, Harry Potter and the Deathly Hallows, the under-age Harry Potter could be traced by the Ministry of Magic if he performed illegal magic. Perhaps even magic has a unique signature.
A current item in Ars Technica, titled Harry Potter and the Serial Number of Doom, by Nate Anderson, reveals some interesting details about digital photo images. An industry standard "Exchangeable image file format" (Exif) data record is embedded into modern digital images.
The article cites an Electronic Frontiers Foundation item from July 20, Harry Potter and the Digital Fingerprints. Prior to the official release of J.K. Rowling's seventh and final book in the Harry Potter series, someone had gone to the trouble of photographing every page of a leaked copy of the book with their digital camera, and they made the entire text available through BitTorrent.
By using an open-source program called ExifTool, the folks at EFF can tell us that the leaker of the book had used a Cannon Rebel 300D, what the camera's serial number is (560151117), and more than 100 other facts about the photos.
Out of curiosity, I downloaded the Windows executable of ExifTool and tried it out. The Windows version supports a handy drag & drop interface, where you can drag an image onto the executable and read a CMD-style terminal window listing of the Exif data.
An image that I had taken with the camera in my cell phone contained the cell phone model number and all kinds of other details, including what version of PhotoShop I had used to clean up the image. Another image taken with my Canon PowerShot S60 contained even more detailed information.
I did a non-scientific survey of pictures in the photo sharing site, flickr, and I couldn't find too much information on a random selection of photos, including my own. I checked out the privacy and permissions settings for my flickr account, and I discovered that they have a global setting that lets you hide your photos' Exif data. When I went back to my account and checked on the Exif data for my pictures, I noted that was just barebones information, such as the image type, size and resolution.