I'm sure the store person is an expert on corys
and can easily distinguish the almost 200 species. More likely, the supplier called them "julii" and the store clerk naturally assumes the supplier knows
I have seen fish labeled incorrectly in stores more times than i can remember, and species that are easily identifiable too. Two local stores had "Corydoras hastatus" recently, in one case they were C. pygmaeus, in the other C. habrosus. The only similarity among these three cory species is that they are all small (1 inch at maturity). I have had store owners tell me that suppliers often ship any closely-resembling fish if they don't happen to have the one the store ordered. Last, as I have repeatedly said, almost all C. julii available in stores will be C. trilineatus; C. julii is not often captured, it can be only once during the year (due to conservation laws in Brazil) and it is not being bred commercially so all stock if they are truly C. julii will be wild caught.
Enough explanation. You have the fish in your tank, sit down and study them using the ID info in our profiles, plus the photos. C. julii are very easily identified by having spots on their heads, like pin-head size, polka dot patterning. None of the other "spotted" corys have this. Plus they are chunkier and blunt-nosed.
Don't take offense, patadams66, this is not directed to you in any way, just my comments on how unreliable people can be, and why they can be.