With all characins, the female is deeper in the body viewed from the side, and noticeably wider when viewed from the front or above, they fill with eggs you see which take up a lot off space. Fully grown, the females also tend to be a little larger than males.
With H. nana, the males are a more coppery colour than females and generally better looking.
With P. maxillaris, you can see through the body to a good degree, (X-Ray Tetra - go figure huh!), anyway, you can see the back end of the swim bladder, (silver bubble looking thing). In males, the swim bladder has a more pointed shape then females.