Best way to tell at this age is the size if they're all from the same group. Biggest will be males 90% time.
Spots all over will usually be males. The juveniles your showing are in such a light color phase it's hard to tell anything. Can you get them in a darker shade?
The best way to tell is spotting on the unpaired fines. I do not see any on the one above, I think that is a female. Females can and will outgrow males when young. Look at any pics of larger males and you will see the spotting I am talking about.
Out of the 11 I originally had, I kept the 4 largest...of which the one photographed is the very largest. All of them have spots to varying degrees. Hopefully the spots on these will spread to the fins. I had heard that only males get spots on any part of their bodies What sizes should I expect to have a more positive identity?
At 3.5" you would see spotting on the unpaired fins. Females will have some spotting on the body like in the picture you posted. I will try and find a pic of a young male. Males and females will get some spots on the body.
These are all the same fish, a female Nicaraguan Dovii. I will find some male pics too. You can see she has spotting on the body and you cant see any in the unpaired fins. She was the fastest growing by far out of a group of 8 or so.
Mine look quite a bit younger... these are only about 3 and a half months old. Maybe they've been growing to quickly for the colors to develop properly yet. I had Firemouths that reached 3" with very little red in the gills and underformed finnage because their coloration and sexual maturity hadn't caught up with the size the fish had reached.