Can't really tell until they start to lay eggs when fully grown.
The Males usually have a lumpy bit on forehead and longer extensions to fins, however it's not a definate rule.
The female will lay eggs (100+) on a flat surface such as slate or Broad leafs and the Male will fertilize them when spawning occurs.
Once a pair have decided to breed they will continue at regular intervals> every few weeks in some cases!!
>You won't find out there gender for a while yet....