I'm going to assume you're referring to the Anime - yeah like MK and Fizz said, Brock is the only one in the whole wide world with the capability to tell Nurse Joys and Officer Jennys apart by appearance.
>Although they all appear identical, Brock is able to tell them apart by little details. This is noted when he can tell the difference between the eyelash length of two different Nurse Joys
:O Amazing. Also, from memory I think in the episode where ash battles in the First Round of the Indigo League, there was a short part where Brock demonstrated his ablities to tell Officer Jennys apart, something about the millimeters of their skirt and perfume.
There is however. apparently another way to tell Nurse Joys apart;
>However, each Nurse Joy will have a different color cross on her hat that indicates which location she works at.
I've never particularly noticed this myself but Bulbapedia and other sites say so :O. So keep observant and look at their hats.