The fins have little to do with it. The easiest way is to look at the patterning on the face:
- Heros efasciatus (green, turquoise, and gold severums) and Heros sp. 'rotkeil': Males will have an intricate red/maroon worming or striping pattern on the cheeks and across the bridge of the nose, while females will have a solid face.
- Heros notatus: Males will have a combination of lines and black spots on the face and gills, while females will have a solid face.
- Heros severus and Heros sp. 'Atabapo' (mouthbrooding severums): Males will often have small black freckles along the gill cover and face. Females may have these as well but they are usually in much lower numbers. In my Atapabo pair, neither the male or the female has facial freckles and I cannot tell them apart unless they're spawning.
The only severum that is not easily sexed by markings would be the super red severums, because both male and female super reds have intricate red spotting on the face and gills.