Well, I may not be right, but male and female bettas can best be differentiated by:
1. Females tend to have a shorter body length compared to the male; and matured females will have a distinctive buldge around its belly.
2. Males nearing its maturity tend to build a bubble nest on the water surface, often triggered by water changes or when a female is present.
I do hope I have answered to your question but of course if there's anything else anyone would want to add please do share.
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