The pictures below shows bettas belonging to Kevin Pelletier. The male will have much longer pectoral fins as compared to the female.
Females flare too but as aggresive.
The female can be differentiated by the body shape, fin length, colour and some special cahracteristics. They usually have a bigger belly, bulging from eggs. Colour are usually not as bright and appears duller. But this is not always the case. They have a ovipositor for laying eggs, which appears as a white spot under the belly.