Identifying DRM-protected songs

I have a large music library (+25k songs) that I play throughout my home using Sonos. The Sonos is not able to play Fairplay-protected files. I would like to convert these to non-protected files but finding all my DRM protected files is a PITA. Is there a way to collect all these DRM-protected files into a single playlist? Maybe a Smart Playlist would work but I can't figure out what metadata will segregate only those DRM-protected files. I have tried the upgrade option and iTunes Music Store offers me no more upgrade options right now, presumably because DRM-free versions are not available.

Any help is much appreciated.

sgtbill