All you can really do is let them pair off as adults and then watch who does most of the on top mounting. It is common for females to mount boys and girls as well, but once they get started in laying, you will be able to tell who is who. Generally the males are more aggressive than the females.
But always keep them in pairs. (you can not colony raise Tennessee Reds.) So I would keep 6 of them. And when they do mature up, you may need to keep the pairs separate during breeding season. Tennessee Reds can be pretty aggressive and may not play nice during this time. They can all come back together during the off breeding season of fall thru spring.