As has been said, you an tell very quickly with true endlers because the females have no color except their base body color while the males show striking colors.
I bet you could sort them at a glance in this picture.
On the other hand, endlers are also sold that are guppy endler crosses. Those can indeed end up with females that are not quite as plain. In that case it is handy to know how else to tell almost any common livebearer male from a female. In that case the anal fin becomes important. I have a closeup of another species that makes the difference very obvious. These guys are tiny so the pictures are a bit grainy looking but the fin shapes jump out at you. This first picture is a female.
This second picture is a male of the same species.