This functionality is not part of Transmission, is part of the Video Player. As I know,
MPlayer can play of videos "corrupted" (in this case, video with only part downloaded), but also depends of the video format as some needs the header only and other needs the entire file.
Then, as BitTorrent and
Transmission download the files in blocks and depending of the rarity, the only thing that you can do is try with one player and ignore if the popup a window saying that the file is corrupted, and if you're lucky with the type of file, you could see it.
Sources: https://forum.transmissionbt.com/viewtopic.php?t=13376&p=60672 http://superuser.com/questions/111405/how-to-preview-unfinished-torrent-video-files