absolutely no way to know the gender if the plant until it is mature. maturity is typically reached after 3-4 weeks of veg growth. some plants will start showing preflowers around this time allowing for you to determine their gender. i will say, though, most cannabis plants wont show any gender until you set them to 12/12 lighting and get them flowering. so you may not be able to find out their gender until you set them to flower and it usually takes a week after being under flowering lighting to show any gender.