Sexing. Preflower Question

So I'm doing a grow from bag seeds, I currently have 14 plants that are about 2 and a half weeks old, 6" tall. I'm wondering if there is a 100% reliable way to sex the plants without having to take clones from each and flower them. I know about plant's preflowers and whatnot, but I'm not sure where and when they present on cannabis If someone would be able to explain this process to me in depth on how to sex my plants, as I want to remove males ASAP so I can finalize my garden layout.

females will show pre flowers when mature, with calyxes at the nodes with two pistels sprouting from each one. there are a few pics on the site showing them. hope that helps mate.