to sex neon tetras the males are slimmer than the females but the easiest way is to look at the blue line. The males have a straight blue line but the females have a little kink about half way along their blue line. Basically the blue line goes along straight, then it kinks up a bit before continuing straight
Its next to impossible to sex neons. Females are plumper when ready to start breeding but just because there's a little kink doesn't mean you've got a female I'm afraid.