TorrentSpy is good software to see what's going on in BitTorrent. It shows the .torrent data in a nice GUI and asks the tracker how many complete copies and downloaders there are. Nicer than my BitTorrent dumper.
BitTorrent has a flaw; it's easy to get 99.9% of a file and never complete because no one has the last few bits. The main use of TorrentSpy is to see if any client has a full copy. Unfortunately it can't detect if the whole file is out there, but no one client has it all. With the average torrent having only 20-30 clients I guess that's unlikely.