In passing if you open a torrent file in any text editor the first line contains the name of the site.
A very slim hope if you use uTorrent and when you remove a torrent from your list you DON'T say 'delete torrent' as well you may find a list of opened torrents in C:\Users_yourusername_\AppData\Roaming\uTorrent if you are using Windows.
But I'm thinking this is a lot of work…