You can try to tell with pre-flowers, but I'm still not even close to 100% and I've been growing for 5 years. The males do show up quickly once you flip. When I want seeds, I pop 4 seeds (good odds for at least one male) and bring them along with my clones. By 2nd week of flower, you know the males for sure. I trim them way back to just one or two main branches and then pinch off all the large male flowers until I think the ladies are ready. I have strong ventilation in my room and I keep the male at the downwind end. Then you wait for one male flower to open (check every day or you'll be sorry) and then mark one or two buds and use a water paint brush to carefully pollinate. This worked perfectly for me on my last grow. Two buds produced 25-50 seeds and the rest of the bud was sensimilla.