Although I can see in real life on darker bodied females when held up to a bright light, it is easier to photograph on lighter bodied females. The best comparison I can think of is looking at a baby chick through an egg shell under a bright light to check for female organs. I hope this helps explain how to identify a female betta a bit better.
Anyway, here are several photo examples of where to look to see if you have a female betta.
The female parts are located where she bars up.
By the way, here is an article that proves that female bettas are capable of sex reversal.
Sex reversal in female Betta splendens as a function of testosterone manipulation and social influence
In Experiment 1, female Betta given daily injections of testosterone (T) for 9 weeks acquired anatomical features characteristic of males as indicated by changes in fin length, body coloration, and gonadal morphology. These findings suggested that a potential for sex reversal exists in females of this species. In Experiment 2 we measured changes in aggressive behavior during testosterone-induced anatomical changes. Aggression decreased toward females and increased toward males as treatment with T progressed. The final displays of aggressive behavior and anatomical characteristics of fish injected with T resembled those of typical males. In Experiment 3, female Betta primed with T injections for 3 or 6 weeks and permitted to interact socially with females continued to display characteristics of sex reversal after T supplementation ceased. Sex reversal in isolated fish injected with T for 3 or 6 weeks was not sustained, and fish receiving only the control vehicle showed negligible change in both the isolated and community conditions. We discuss the results in terms of similarities with the sex change process found in isolated communities of coral reef fish.