to illustrate what I was looking at see pic below
a girl

a boy


see the difference in profile shape? - that is the best way to differentiate when they're juvenile, for adults it is that, the size (females are much bigger) and nuptial pads for boys - black spot on inside of his thumb, very similar to those for pacmans, but those are visible only when they're in breeding mood. I had somewhere a pic, can't find it now.