Newspapers themselves were not that important in fashion until probably the mid-to-late 1800s, at least in the U.S.

Instead, fashion plates were all the rage. Fashion plates were drawings of women and sometimes men (but not usually together) wearing what was considered high fashion for the time. Like early fashion magazines, really. It was these fashion plates that were purchased and distributed around Europe and the United States in order to keep people "up-to-date." The fashion plates generally originated in England and could be ordered.

As far as how things worked before the fashion plates, well... how do they work now? Sure people who are especially fashion-conscious or consume a large amount of internet and print culture probably know what's "in," but a lot of the info the average person gets is just from seeing the people around them. Fashion spreads around social groups in order to become a trend, so if you are hanging out with your friend Lady Elizabeth and she is wearing some crazy new hat and you like it, you'll probably get one too. You also might get ideas from the "media" of the time, such as art or plays. But mostly you're going to just observe the people around you to stay current.