Its hard to sex Severums when they are young, the slowly develop indicators as they grow. I have a female that we could tell when she was 2" long, and a male that we weren't completely sure about untill he was over 3".
Basically, as they mature, males will develop a more pronounced, square shape to their head and their dorsal and anal fins become long and pointy, sometimes reaching the length of their tail.
Females do not develop either of these characteristics and their dorsal and anal fins stay short and rounded.
Either way they are beautiful fish, they have a lot of personality and love to entertain!
Hope this helps,