Oh good lord... I've been meaning to write a story about this and now I guess I will. If you've got, for example, 10,000 people living on an open plain with no defensive properties; they smelt ore but make no thrusting weapons, they all live in the same size dwellings, with no public buildings, and analysis of remains indicate that they all had access to same type and amount of food, they create all sorts of art that depicts beauty and nature (in contrast to later pottery and art from other places that depicts kings standing over slaves in chains) - then we know all kinds of things about what that culture was like, what it valued, the social organization, gender relations, etc. Examination of art and pottery alone tells a HUGE amount about various cultures, and that's before you add in all the rest of it. You could say it's an educated guess, but it's a very, very highly educated one that doesn't really reasonably fit any other conclusions.