Detect invalid songs

[Phrogz]

1. Create a playlist and add a song to it.
2. Remove that song from the MPD library.
3. Load the playlist.

The playlist now has a song with a file but no other data. How can I detect if this is a valid song with no metadata, or an invalid song?

I'm currently using the heuristic of checking to see if song.time.nil?. It would be nice to have something like MPD::Song#valid? to see if the song instance is grounded in reality and able to be used.