Not quite sure, even though I have two of them myself, but I think the females are slightly larger than the males. Also, viewing from the top of the tank downwards, I have heard the females, when with eggs, are rounder-bellied.
As soon as mine decide to grow bigger, I'd probably be better-abled to answer your question....
From what I heard, the males will be thinner and brighter, and their red will be more intense. The females will look like pretty big, almost like they ate too much.
They're cute, aren't they? I've never had them, but from what I've heard they are quite hardy - no, stubbornly hardy - and do better in groups. Here's some info:
On a hardiness scale of 1-10 they rate at least a 20 . Our first attempt at fishkeeping was about 4 1/2 years ago and we wanted a cold water tank (we got a cheap 2footer at garage sale) but didnt want goldfish, well the LFS guy suggested minnows as they dont need heating inside during Tasmanian winters. Well we got some suitable plants and fish took them etc. about 3 months later we had a house fire fairly limited direct fire damage but with scorch/smoke damage throughout the house. as it happened the cord to the filter was destroyed and since the tank was black and only 1/2 full we figured the fishies was gone too . 3 weeks later after fire/insurance investigation is done we get to go back in and clear what the insurance isnt covering get the tank to our new place figuring all the fish are dead, it then that we noticed one of the little buggers had made it , I dont know how he made it through 1/2 the water being cooked away and REALLY crappy water but at least he had his friends for food