Communications associated with illicit activity has become a new focus of advanced communication technology. A steganographic method of binary image, which embeds data in facsimile by modifying the runlength of black pixel can build a covert communication over fax. This letter presents a steganalytic scheme to detect the message embedded in facsimile using run-length modification method. The detection is based on estimation of the original run-length distribution from the stego-facsimile, which is done by filtering stego-facsimile with corresponding filter. Experimental result indicates that relative modifications as small as 10% of usable run-length can be reliably detected