I don't know how to ask iTunes a song is included in any playlist, but it's easy enough to set iTunes up to sync only your playlists. Search iTunes help for "sync iPod" and select the help topic "set up syncing for iPod, ...". When the help page opens click on "Select items to sync automatically" and follow the instructions there. Remember to check "Sync only checked items" in the summary pane. Click apply, and your iPod should sync to include only the music in the playlists you want to keep.