How to select photos which are not tagged or set face(s) in Photos? In Photos there are not a selector called "Face is empty" or "Tags is not set", so what should I do?
As of April 2016 Photos does not work like that.

Although it sounds like a limitation, it actually makes sense. The program detects "candidate" faces and presents them to you to classify them. After learning about a few of those faces, this opens the door to reassessing pictures that previously would have been deemed without faces.