Is there a way to rescan for faces?

No, there is no way to automatically do that. If you migrated (not the same as imported) an iPhoto Library into Photos when you started up Photos, any faces identified in iPhoto would have been migrated as well, but Photos has to have some identified faces for anything to display in the identified section of the Faces album.

Photos supports face detection in the same way iPhoto does. It attempts to identify face shapes in photos, but it is up to the user to decide if they are real faces & if so to tell the app whose face it is -- just as humans can identify faces but not know what name to associate them with without some other point of reference, no app can do that.

So in effect, the "Suggested Faces" feature is more or less equivalent to a human asking another person what their name is -- once that is known, it becomes easier to associate the same face with the same name when seen elsewhere. Once you start adding names to faces in a few photos, the Photos app will ask you if faces in other photos are those of the same person, & if you confirm that they are, will automatically add those photos to that person's group in the face circles in the upper part of the Faces album. But that's the extent of the automation -- it just adds photos to the list for that person.

Each photo is scanned upon import.

However, if for whatever reason it's not (or for example, if you deleted faces originally and would like to re-scan photo) you need to edit it.

The fastest way I could find is to select all photos you want to re-scan, and do "Rotate clockwise" and then "Rotate counterclockwise". That will cause photos to re-scan photos you rotated for Faces.

Good luck,

Dan

Right click the photo and select Get Info

Any detected faces will show in circles towards the bottom ... to the right of the list is a + sign to add faces, if nothing's been detected just click the + and drag and expand the circles to cover the face, and add the name which should autocomplete.

just click the + and drag and expand the circles to cover the face, and add the name which should autocomplete.

The manually added faces circles are second-class. Photos will not use them to recognize a person. I would not start naming faces in a photo until Photos has had enough time to recognize all faces automatically, that it can detect. I wait a day or two before I name the faces in recently imported photos and let the mac run over night to give Photos enough time to run the background processes for faces detection.

You could use Face Crop Jet App for Mac OSX.It can detect and crop faces from photos in Bulk.