If you really want to be sure your het is in fact a het buy from a reputable breeder and/or make sure to get paperwork of the genetics of the animal your buying.
I know of a couple of reputable breeders that don't send paperwork with hets.. So don't be disappointed if you don't get any. Just research your seller and go from there.
When you've got 10,000 people trying to do the same thing, why would you want to be number 10,001? ~ Mark Cuban