How can I force Photo app on the Mac to scan for faces in a given picture?
I have noticed on El Capitan Photos that manually adding faces on individual photos (cmd+i, click circle with + , position manually and resize circle, type person name) then skipping forward one by one (right arrow or slide) will rescan sequential photos. 

Then going directly to Faces and clicking person face which you were working on will suggest that more photos could be added.

This made Photos to gain additional 10-20% face recognition for me.