How do you know if you are getting authentic DePorcyline Fleese blue delft?

I went to the factory and they warned us about fakes. There is a town called Delft so all sorts of pottery places can call theirs that. I know it has to be stamped with a blue jar. Sometimes if it is priced too low that would be a good indication because it is not cheap but so beautiful! I wish I would have picked up more on our trip there.