Write that information on the back of the photo with a soft 6B drawing pencil, which is available in art-supply shops. For home movies, write the identifications on the leader. Note when it was shot, by whom, and what the event is. Home movies can be very difficult to identify. If possible, sit down with the person who made the movie, ask them to narrate it, and take notes.
Many people have photos in their collections that are undeniably old, but are unidentifiable.
You often can't say with certainty whether the person shown is a family member. Set these pictures aside and work on them last. Put your energy into the ones that can be identified. After you've identified the photos, work on storing them properly. There are two primary ways to store photographic prints - using a filing system in archival boxes or using photo albums.