I was 5 full weeks through vegetation and I could not see sign of sex at all. I guess if you veg for like.. ever.. you will eventually maybe see hints of the sex (preflowers), but um.. typically, people don't veg much longer than they have to, thus, you'll find out in flowering. Once you change your light cycle to 12/12, your plants will take a few days to change over to the flowering stage. It's a chemical process, which is why it takes a few days.. after those few days, the plant will burst upward (in my case) and I was able to tell sex of my plants after a week into flowering (don't be confused.. every day your plants are on a 12/12 cycle, they are considered "flowering" for all intents and purposes