That's not cool that the rep at the petstore couldn't give you good info or atleast find someone that could.
If there are many similar looking cichlids it can be a little difficult to tell; especially when they are juveniles. JD's don't have as much color when they are very small and young. I recommend just google Jack Dempsey and look at photos, mainly look at juvi JDs. This should give you a good idea.
also, maybe you can ask someone at your petstore if there is someone else that works there that is more knowledgable about cichlids so that you can make a time to go in and see that person.
Honestly though, you should find a different fish store. That is terrible customer service.