There is no good way to do this. Because the BitTorrent protocol downloads pieces in arbitrary order, there is no guarantee that the part of the file necessary for previewing (usually the beginning of the file) is present. To further complicate matters, some torrents are packaged as an archive, which would be quite difficult to extract until it's complete.

Still, if you want to attempt to view the file periodically, you may eventually get lucky. First, make sure the file you are downloading is not an archive. If it's a ZIP or RAR (R00, R01, ...) file, forget it. Next, you'll have to interrupt the download, since BitTorrent locks the file in an exclusive mode until the file is complete. You can now try opening the file in whatever application is meant to be used to view it, but don't be surprised if very strange things happen. Finally, you'll want to resume the transfer, unless you've determined that you no longer want the file.