I would be quite surprised if there were WC dobrogicus in the trade. Seeing as how many europeans breed them it would be pretty stupid to resource to catching wild animals which would be ilegal since the species is protected.
If you acquire the animals from a breeder, you will be a 100% sure that they are CB.
They are easy to breed, they just need a winter period at temperatures lower than 10ºC, and a healthy diet.
As for distinguishing T.d.dobrogicus from T.d.macrosoma, the best clue is location (this species is very variable in morphology and coloration due to genetic introgression with other Triturus species). If you know where the animals originate from, you´ll know the subspecies. Most, if not all, european breeders should know what subspecies their animals are so that´s not a problem.