You cannot tell if a snake is a het. If the breeder you got it from produced that het pied, he/she should know and tell you that it is. If the breeder did not produce the snake, and says it's a het pied, the only way to tell if it's a het pied is if you do this:
1) Breed the snake with another het pied. See if the hatchlings are pieds too, and if not, your snake is not a het pied. Make sure the snake you breed it with is a het pied, or else your hatchlings will all look like normal BPs.
2) Breed the snake with a normal. After you have the hatchlings, raise one of them (of the opposite sex) and breed it back to yours. Though, you may not know which are hets and which are normals in your clutch because there is no way to tell unless all babies are bred to the parents or each other.
3) Breed the snake with a pied. If your babies come out looking like all normals, your snake is not a het. Though, your clutch would be het for pieds.