Photos: http://imgur.com/gallery/9NzHBnG
Are there clues you look for? Do you consult other historians? What sources would you compare against to check your hypothesis once you think you know when/where they are from?
Prominent clothing styles such as the cloche hats and Louis heels that the women are wearing are a great indication that this photo in particular was taken in the mid 1920’s. The pants the man is wearing are archetypal of the male collegiate look popular of the time as well! The quality/genre of the photograph itself is also a great indication.