well, I believe venting is the only way... I know small... jk I am not sure, but if it is big enough you can vent... I know in small fish you can only see the one large dot with the females...
Venting.
If you start with a few (who came from the same batch) you can get a pair to form naturally and the pair will consist of one larger individual (male) and one considerably smaller (female) but if you only have a few to choose from you really need to vent to be 100%.