It's not hard. You know where you're starting and where you're ending up, scroll through that path and then some more to either side. East-west routes in the US tend to curve to the north (it's a bit shorter to go around that way). I haven't flown enough north-south to know where they tend to curve, but the Boston->Atlanta leg on my flight out started out going almost due west for a while, and stayed inland, so that's my first data point.

For most northeast-west coast trips, I already remember the route anyway. I knew SF->Salt Lake City (first leg of yesterday's trip) was gonna mostly follow I-80 without having to think about it.