Ther is no "for sure" marker. What most people are refering to are black striping on the belly of the animal. The belly of the animla will have black stripes border the white belly scales. These stripes are generally on the bottom on third of the animals belly. However, plenty of people, myself included, have 100% definite Hets that do not have the marker. Having said that, it may be something that you could use when picking out Poss. Hets., but there is no "for sure" marker.