A document authentication method uses a watermark added in a printed document to detection possible alterations made to the document after it was printed. First, a visible watermark in the form of a dot pattern is overlapped with an original digital image. The watermarked image is printed out as a halftone image at a first resolution. The watermark in the printed document appears as a light gray shade. Later, the printed document is scanned back using a grayscale scan at a resolution higher than the first resolution. In the scanned image, altered areas would appear flat (lacking intensity variation) whereas unaltered areas will have relatively large density variations due to the watermark dots and the fact that the image was halftone printed at a lower resolution. Alternations are detected by identifying flat areas within the image using a combination of flat block detection and a multiple thresholds method.