From the top of a mountain, we can easily see the other peaks afar. And from the visibility and the orientation of the peaks relative to each other, it is easy to conclude that the Earth is a sphere.
We can do it by getting the distance from the observer to the peaks and their heights. From the data, we can draw a geometrical diagram or even a simple physical model using the flat-Earth model. It is easy to conclude that the visibility of the peaks in the real world does not conform to what is expected in the flat Earth model.