Isometric Depth Sorting

[lilspikey]

Hi,
  I've been making a very simple isometric sprite generation tool, which works a bit like placing lego in 3D.
  Anyway, I have a slight problem with it in terms of depth-sorting the polygons as I rotate the created objects.  The polygns being sorted are faces of the lego-like blocks, and so are all reto-linear, which should mean that a sort order can be found.
  Can anyone give me any hints on how to determine if two isometrically projeted polygons are in front or behind eah other?

  cheers,
     John