The preview button will often allow you to see/hear the bits of the video/music downloaded so far.

However, over time we learn to identify and avoid downloading a bunch of corrupt and incomplete fakes. Often the spammers are on T3's, and the files are the first to show in many results. The Preview button will not work on those files, since the contents are not really reliable.

Apparently many of the files on the network by Kazaa users were as you described--the middle of the file was garbage. Since many Kazaa users have switched to the gnutella network (LimeWire uses gnutella), we should be seeing more garbage being shared.

You can also use the Bitzi lookup (right click a search result to get at the Bitzi option) to see if anyone has reported that file before you download it.