Method of and system for detecting a watermark in MPEG-2 encoded video data during high-speed copying of the video data. A bus spy (120) selects independently coded frames, such as I-frames, from the video data and feeds the selected frames to a watermark detector (130) for detecting a watermark therein. By only supplying some of the frames to the watermark detector (130), the average input data rate for the watermark detector (130) is much lower than the data rate during the high-speed copying. Various embodiments present various ways to efficiently supply the contents of the selected frames to the watermark detector (130).