Digital video watermarking has been around for a long time. I would imagine that most digital video is now watermarked. If so, Youtube probably just detected the presence of a watermark, and determined that the video was not user generated. Youtube could check for the presence of a watermark using only a couple of frames of video. Have a look at this article talking about video watermarking: http://www.kodak.com/US/en/corp/researchDevelopment/productFeatures/cinema.shtml Kodak is currently the only company to publicly announce and demonstrate (at ShoWest in March 2001) that its watermarking technique can hold up in real-world testing; that is, the watermark can survive the actual process of capturing a projected movie with a camcorder. Moreover, the amplitude of the Kodak watermark is such that it is invisible to the viewer in the theater, thus maintaining the image quality that is such an important part of the movie-going experience. In the ShoWest demo, 16 bits of information were embedded in each frame of the movie, and it took less than 15 frames (0.5 sec) of captured video to extract the 16 bits with 100% reliability. Often, the watermark was successfully extracted using fewer than 15 frames, sometimes only one or two frames.