How to find photos not in any album in iOS photo app?

The best solution is to use the system as the developers intended, despite the inconvenience.

For Albums created on the iOS device using the Photos app (I believe the same applies for Events created in iPhoto on iOS):

Deleting a picture from an album removes it only from the album. Deleting a picture from the Camera Roll deletes the photo from the device—but it may still be part of your Photo Stream. Deleting a photo from Photo Stream removes it from the stream, but will not remove the photo from devices on which it has been included in Albums or Events. ᔥApple — iCloud: How to delete photos from your Photo Stream

There is no magic incantation that will reveal photos you've deleted from albums.