DG9J is only formless when executed to the highest of levels, ie, when it transcends the actual teachings of DG9J. But DG9J itself actually is composed of many unimaginably brilliant movements and attacks, which is why the various stances have literally hundreds and thousands of changes in the general index stance, which Linghu Chong (genius that he is) partially learned in one night. So it's very conceivable that people can recognize it, as Fang Sheng definitely did.