If you 3D Touch press a a existing folder a bit longer, you can see all Apps that have a Notification from within that folder.

3D Touch a Folder

3D Touch a Folder

That is a great 3D Touch function, if not one of the best yet! And honestly, I didn’t know that until yesterday.

All Apps with Notifications

All Apps with Notifications

You need a Apple iPhone 6S, or iPhone 7 do use 3D Touch.