how can i find which playlists contain a specific song
I have dozens of playlists and would like to find out if I have any songs that appear on more than 1 playlist. How can I tell which playlists contain a particular song
For a single song, right-click on it and select Show in Playlist. This will display a list of all the playlists of which the song is a member:
In this case, the selected song is in three playlists: "Unplayed Music", "Popular Music" and "All Music". The last two are contained in a playlist folder called "Everything", and the last one is a Smart Playlist.
If you want to see this information for more than a single song then you can create Smart Playlists whose contents are based on the other playlists that songs are in or are not in. With a small number of playlists this is manageable - with 10s or even 100s of playlists it becomes cumbersome (at the minimum). There is (unfortunately but understandably) no function within iTunes that allows you to generate a cross-reference listing of songs with all the playlists that each is a member of. It may/should be possible to do this using an external/scripted solution that generates this info by interrogating the iTunes database.