My favoured stretch of test route follows the
shore of the St Lawrence River where it is quite wide (width > 2 km - meaning there's little elevation change in the area). I've got approx 1.6 km of recently paved, low-traffic, 2-lanes with no more than a few feet elevation change along that length.
In searching out this route, I discovered how difficult it is to estimate what's flat and what's not by just eye-balling it. I thought I could use a few other roads closer to the city, but it turned out they were not flat at all when I compared my first few runs in each direction.
I think the contour map/Google elevation map idea is sound.