With the price of 100% Het Pied female nowadays I would not even bother with possible hets.
Now as far as markers I have successfully relied on them to hold back poss hets that I had produced years ago (keep in mind back in 06 a het pied female was $1200)
Here is what you are looking for
Those train tracks can be very variable and not always as define as those in the picture (this is actually a DH Albino Pied)
Now are markers reliable? Not always I have seen normals with similar markers as well.
Finally in most cases there is also more to it then just the belly markers, after you see a few het pied and possible het pied you will notice that the patter as well as head are a little different than just a plain normal.