You can sex by checking the pre flowers of the plant, by the nodes. First your calyxes will developed then your pre flowers. Males will always go into pre flower so check for your sacs anfpd females you will see the smallest thinnest little hair or 2 coming out. Preflower usually happen 2 weeks before they are old enough to go into flower..example 8 week veg, you will be able to early sex by checking pre flowers at 6 weeks.