Females have more of a round shape to their vent configuration. Males are more elongate, pear, or teardrop shaped. Females are usually pink and swolen around spawning season, males are not. And lastly, healthy mature males will emit milt near spawning season with a light squeeze.
This may sound strange and trivial, but I give a difficult fish a little squeeze around the vents. A male will usually squirt urine, females won't.
