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I think as a general rule of thumb, the males tend to have more vibrant, bigger dorsal fins. Unless the fish are really young, identifying a male and a female from the same species should be easy. There are cases though.
I had a 'Tefe' female and a 'Double Red' female in a tank and the double red began to flash the tefe. The tefe didn't really respond to it but identifying fish by behavior may not be 100% dependable.