Thick black parallel lines running down the sides of the belly - usually at least the bottom third of the snake if not more.
Clear white belly at least three white scales wide (between the solid black lines)only exception would be if your snake also has an orange ringer which is also sometimes found on hets and would start in the belly area.
This might be a het pied marker:

This would not:

-----
Tosha 
JET Pythons
(CJBianco explaining the origins of the BP market.)
"In the beginning Bob created the Ball Python market. And the market was without morph, and wild traits were upon the industry. And Bob said, Let there be morph, and there was morph. And Bob saw the morph, that it was good; and Bob divided the morph from the wild trait. And Bob called the morph Albino, and the wild trait Normal. And the Albino and the Heterozygous Albino were the first investment.
-- Christopher 1:1-1:5"