I have always had difficulty with properly sexing my wild bettas, particularly fish from the coccina group.
In some of the species, the differences between the genders is obvious, but there are other species such as my Betta sp. cf. rutilans green, where the differences are much more subtle. I could probably spend all day comparing my rutilans and still get it wrong.
It's like when I read articles on how to distinguish between closely related species, and there is mention of counting rays and looking at scales. I sit there wondering how on earth these people are getting their fish to stay still long enough to even get to the 'greater snout length' stage. I have to cram my fish full of blackworms so they are too heavy to swim away if I want to get photos of anything more than their retreating backsides.
Of late, I have been using ventrals to guess the genders of my juveniles. I have found that even from a young age, males will show much fuller ventrals than the females, whose ventrals are generally fuller near the body before tapering down to a very fine point.
I also compare the fin sizes of fish that are of a similar age. The above female, is roughly the same age as her two brothers, both of which are already displaying fins that are much larger than hers. Apart from that, she also has a shorter, and much straighter anal fin than her two brothers, and visibly blander colouration.
I have come to realise that simply looking for an ovipositor is a poor way of sexing the coccina group species. Most of my males show 'false' ovipositors (even my mature males) and even with a bright torch, the fish are too dark to properly spot ovaries.
Fortunately, I have found that appearance tends to be fairly uniform amongst the juveniles, so that does make comparisons between individuals a lot easier because there is little variation to throw me off.
So far, I do have to say that I think I am improving with distinguishing between the male and female juveniles. Unfortunately, while it is easy to pick out the males even when still very young, I do still struggle with telling the difference between a male that hasn't quite sprouted yet, and a female.
Hopefully, as I start breeding and raising more fry, the process will get easier. Otherwise, I am going to have to wait until my fish are literally spawning before I can sell them as guaranteed pairs!
Hailing from Melbourne, Australia, I currently keep and breed a number of species from the coccina complex.