After you've done them, I noticed if you haven't made any mistakes, the level-hex in the menu screen will have a diagonal line crossing it (from top left to bottom right, sort of), and if you've made a couple of mistakes but completed the level, that level hex will be blue but lacking that diagonal line.

However, if you've made 1 (one) mistake before completion of a level, it also shows up with the diagonal line looking exactly as the perfect-0-mistakes ones.

So, in order to get the perfectionist achievement, are 1 mistakes per level allowed? Or is there any other way to tell which levels where you've erred once need to be redone, other than actually writing them down, or immediately redoing them to perfection every time before moving to the next?