The present invention is concerned with detection of a “fragile watermark” in a printed image to aid in a determination whether a document under examination is an original or a copy. The watermark may be applied by phase-space encoding data to be included in the watermark. The image may be examined on a block-by-block basis after scanning. Correlation of detected watermark strength with block brightness and/or with wave vectors used for encoding may be used to detect that the image is a copy.