colloidial silver can be sprayed on the plant during the first few weeks of flower if it is a female and it will produce male pollen sacks with fem'd pollen.
if you grow regs you wont know the gender until you place them in 12-12 lighting or similar. so it is recommended you take clones prior to sexing your regs. if you want you can also flower them without taking clones, then grow the females all the way through harvest and leave the bottom larfy bud after harvest, turn the light back to 18-6 schedule and the buds will start to grow new vegetative growth that can either be cloned or reflowered for a second run. this is called monster cropping btw.
edit: monster cropped plants will tend to have only three fingered leaves for a while but will grow out of it and the three fingered leaves will yellow and fall off eventually
here's an example of what that looks like, this was a clone taken from a monster cropped plant and the yellow leaves are all the oldest 3 fingered ones so it isnt a tell tale sign of nutrient deficiency. more like season change
and here the plant is a few weeks later.