Some plants never show sex until flowered and I've never had one show until 5 or 6 weeks at the earliest.
If I really want to know I'll wait until it has about 7 nodes then top it above the 3rd or 4th node and set it to clone under 12/12 light with a 2700K CFL or LED bulb in a closet away from the grow room. If it's already sexually mature I should know the sex in about 10 days then can switch back to 18/6 or 24/0 to flip it back to re-veg if I want to keep it or if a male and I want the pollen keep it at 12/12 and collect the pollen to make more seeds.