Does the suspected male have an ovipositor? The best way to sex bettas is not by their physical traits, since there are short finned males and long finned females, nor by their behavior, since some males and passive and some females are rabidly aggressive. The best method, though there are even some exceptions to this, is to seek out the small, protruding white 'dot' on the underbelly of the fish, close to the ventral fins. Almost all bettas with an ovipositor are females, but some males for whatever reason do manifest genital deformities that look much like an ovipositor (though the "dot" is usually much less pronounced than in females).
