Since it was purchase in a pet store chances are it was a fresh CH import.
Stores such as Petco, and Petsmart get CH each year.
Now if it came from a small pet store it could have been a CB however there is no way to tell the genetics of the parents, and even if you could it would not matter in most cases, if it is the product of Dom or Co-dom X normal all that makes him is Co-dom or Dom sibling in other word just a normal which will have no bearing in your future projects.
Some breeders also sell their poss hets to pet stores sometimes (I have in the past) however again it would mean a chance of being a het, and to determine if it is or not and for what it would take YEARS to do so, and at the price of hets those days, not sure if it would be worth the time and effort.