BITD before the internet, Google maps and Strava, we used (free) gas station road maps to plan routes. The ones from Rand McNally were the best because they were very consistent in how roads were mapped, related to their type and traffic load. We judged terrain by clues like rivers, roads crowding into what seemed like a narrow valley or pass, how wiggly a section of road seemed to be, etc. Many of actually got very good at this game and had a good sense of what to expect when we got there.

These days there are so many sites that show elevation profiles, and you can often get detailed profiles by mapping only a short section of your route which may be suspect. You can also use Strava to get a sense of where the locals ride (don't ask me how, I don't Strava). Otherwise use the same types of clues we used back in the days of paper maps.