When you think you have a tile nearly flat you can find out if it is perfectly flat by right clicking>flatten. You will then get the message 'The ground is flat here' in the event window.
A better way to check <b>NEW</b>: You can see the flatness difference in height between one side and the other of an edge of the a tile is to press 'c' key, then look at one by using examine on the tile and if the line is straight, then you are sure it is flatborder.
==Tools used==