Only with casual observation and lack of really thinking it through. As a child we traveled twice yearly east to west and back again for the family business. I'd notice that from far away the mountains appeared shorter and as we got closer we could see more and more of the base of the mountains. It puzzled me but at five years old I couldn't form the ideas to ask questions. Then, again, as we passed a certain point, the mountain range would foreshorten as I looked up. In other words, I'd see more of the base but the top elevations would seem compressed. Visual illusions but what really puzzled me was why the base would be hidden from a distance. Grandpa would take us up in his business airplane and from higher elevations you could literally see "flat" areas curving away into the distance. In school looking at globes it became obvious why this is so. Then there's the problems associated with surveyors making grids of property. On small lots they can easily mark out perfectly square grids for property but on larger grids, they have to fudge it. Eventually they can't fudge enough and have to make some plots not square. Surveyors knew of this problem and why it existed. Any thinking person with eyes can see this easily. Watch sail boats go over the horizon. Obviously the ocean curves. You can SEE it. If you can't, you're stupid and fail to observe what is obvious.