If you're a perfectionist when it comes to email organization, you may want to make sure that all of your messages have been organized. It's easy to do a search in Google Mail for all of the messages contained under a certain label. But how can you find all the messages that don't have a label/folder?
First, turn off conversation view (Settings -> Conversation View -> Turn Conversation View Off). This will keep messages from being grouped together in conversations where some of the messages might be labeled.
Second, copy and paste this string into the search box, then tap the Enter key:
This will find all of your messages that don't have a label AND that aren't in your Inbox, Drafts, Chat, or Sent folders. From here, you can go through and file your messages to your heart's content!
I first learned about this technique from this article: https://it.umn.edu/finding-all-unlabeled-mail-in-gmail