To identify all songs which no longer connect to file, simply go to the music library in iTunes, Select All, click on the list and move slightly as if dragging the songs to a play list. Hold the listing until a red oval appears showing the number of files, then let them drop back in the library. Presto, every song that is orphaned will appear with the gray statement mark to the left.
Now if you could only sort by that column to quickly eliminate all orphans!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061206193107249