I've owned several normals in the past, including a few females, that had a perfect 'piebald' marker on the underside of the tail...

As they came from small, hole in the wall petshops, (with cheap prices) I am absolutely certain they were not hets, especially the girls (this was a few years back, when females from pied litters, even possible hets, commanded a rather large price).

Without papers, I would be very cautious...as I think the 'pied' marker happens in many wild type balls, and really has nothing to do with the pied mutation itself.