hiya,
you might find that if they are locally tank bred, you have a leaning towards one sex or the other, as ph has an effect regards % of males to females in any given brood.
there is some difference in the patterning, due to regional variation, suffice to say all males dont have spots and some females do.
the males as i said is more elongated with a pointier dorsal fin, and spear shaped tail.
If you are sure its a female , take it back- or relocate it to another tank.
go and study a tank full, and you'll be able to pick out the longer larger specimen, more likely to be males.
dont rely on the LFS guy. use your own discretion, and say "I want THAT fish"!! make him earn his money......