You can't tell the difference. In a Het X Het breeding people should refer to the normal looking offspring as 66% hets or possible hets. They are normals until bred and proved to be hets.
Think of an allele, which a a pairing of genes. To create a Piebald, you must have a pair of Piebald traits, we'll use "P" for the Piebald trait. "0" is a normal trait.
(Both Het. for Piebald)
Snake A______________Snake B