Try looking at the ventral fins. Sometimes though not always, you can tell by the way they fall. My females ventrals curve backwatds towards the tail. My males hang straight down. Mind you, I know which ones are females aLready, as they are paired, and breeding. My males are also developing a slight nuchal hump on their heads, and the females are rounded under the belly with the eggs. These differences only showed up once they matured, so keep that in mind, and it doesn't work every time.