Eugh, don't even get me started on that one!
Basically, if you have room to buy a group to allow them to pair then you can do it that way. (And re-home the rest)
You can vent them, literally pic them out of the water and see if their 'vent' is bigger than their anus, that would be a female.
You can wait until they are 100% mature, if they show no characteristics of being male and at least a couple of female ones then you know you have a female.
Or, go down the route I probably will if I want one. Get in touch with a supplier who has breeding females they're willing to sell to you. I found one if you would like me to PM you?
However, in the LFS when they're around 1-1.5" then you can tell some are male sometimes....but the rest could be female or adolescent males....
I'd convinced myself that my boy was a girl because he was quite a late developer
, or perhaps I'm just impatient!
Edit: Due to my sleepy brain making me forget how to type