What i have noticed, its that the females are smaller than guppy females (in general). They seem to have much more muted colors than female guppies, too.
Flutter,
endler females look exactly like female guppies. There is basically no way to tell them apart if you are not trained and sometimes you still have problems. I have found that my endlers tend to be smaller than Wild Form Female guppies, but not by much. Female guppies tend to be more grey with a blushish sheen on them or even a yellowish sheen, while Endlers tend to have a greenish sheen.
But I doubt you have a female endler. Most stores and I mean 99.9% of them do not get female endlers. I personally feel its do to control of the breeders. Why sell a fish that breeds like crazy and not make a profit off of them. Females will breed like crazy if given a chance. I have given away dozens to my friends. It is by fare easier to by the males instead of females. If you want to buy endlers buy them from a reputable dealer/breeder. You more than likely got a guppy.
Here are some photos of female endlers.