There is a long history in China of forged artwork, and these days it has become a big business, with "painting factories" churning out cheap imitations of great works. Cheng Man-ch'ing's paintings have, along with those of hundreds of other artists, been forged, often quite ineptly.
A forger copies the format and content of the original painting (in most cases working from a printed version), but unless the forger is highly skilled, the net result is nowhere near the work of the master. In this example of a forgery (left), Cheng's style is superficially copied in terms of layout, content, and subject matter. However, we can see that the forgery has missed the mark: it is flat and unexpressive, the plant has no life in it, and the parts of the plant do not relate to each other. When compared to an original work of Cheng's (right), we can see that the authentic work is imbued with qi, and that everything--composition, line, detail, brushstroke, color--"hangs together" as a whole. The plant looks alive, as if the wind has blown it into place just at that moment.
What is missing in most forgeries is the indelible mark of the artist, the focus of the mind and execution of inner expression, sponteneity, feeling, depth, skill, and technique: the power of brushwork, strength of line, facility in ink tones, and an intimacy of understanding the interconnected brushstrokes of both painting and calligraphy.
Ironically, forgery in Chinese art is to a certain extent a byproduct of study. An aspiring artist is expected to master the styles of prior great artists, and gradually work towards development of his or her own style. In fact, Cheng Man-ch'ing's friend and colleague Zhang Daqian (張大千 Chang Ta-ch'ien, 1899–1983), one of the great figures of 20th-century Chinese art, took great delight in creating such skilled forgeries that he was regularly able to fool experts. Nevertheless, forgery remains trickery.
2/2017 Update: Readers can find a comprehensive article on Chinese art authenticity, connoisseurship, and artist Huang Binhong here in Chinese Heritage Quarterly.