I am intrigued by this as well. I think they check for the existence of certain primitives and a sequence in which they are drawn. For example for level 1.1 where you need the 60 degree angle, if you use Explorer mode to view the solution and then you draw a "cheated" line on top of it, it won't give you a completed level because you don't have the sequence (2 circles and an intersection point). It also doesn't allow you to draw another line on top of an existing line, so you'd have to restart the level in this case.