Here is a nice link to NWE, showing photos of the black "tracks" that are associated with "het pieds"
http://www.nextworldexotics.com/hghpm.htm
As someone else mentioned already though... a het, is only as good as the seller you buy it from.. Paperwork means absolutely nothing.. I can make you paperwork for a het pastel and sell a normal with it.. etc. etc..
I have normals with these "het pied" markers.. and I have seen 100% het pieds with no "markers"..