Sounds complicated, but isn't.
When you have a large torrent consisting of many files, and you select only a subset of them for download, you end up with many incomplete fragments from adjacent files. In order to work out which ones are the ones you selected and are complete, you have to work through the original file list comparing manually which ones you selected with what was actually downloaded. This is really tedious with many files in a torrent.
A simple solution would be to (optionally) mark the completed and requested files using a Finder label, so they immediately stand out in a distinct colour. The label should apply to the original file system entity selected in the torrent, which might be a folder or a file.
Auto-label completed, requested fragments
Re: Auto-label completed, requested fragments
Have you tried checking the preference to "Append .part to incomplete files"? That makes it obvious which files are incomplete. The odd thing is that I sometimes get complete copies of files that I did not select to download.