I never knew that you can predetermine the sex at such an early age.
Well, its definitely not an exacting science, that's for certain, however, I do whatever I can to increase my chances. After years of experience, you can almost tell the differences with a good deal of accuracy.
Sometimes, they will pop pre-flowers in the 3rd to 4th weeks, and if you look close enough, use your photographers loupe that you use to tell if your buds are done, to see if the pod has a little slit in it and sits on NO stem and isn't round but, more ovule shaped... that is PROBABLY a female, but you won't be able to tell for sure until it shoots out a hair. (i've seen them like this and they turn out to be frickin males too, more times than not, they're female though)
Like over the years, I've noticed that if I run my hand over a male plant, vs one that is possibly female, the female will be a bit more pungent, and your hand will come away a bit more sticky...
In general.... the male will be taller than the female and lean towards the female..
Males will also "Stretch" and won't have as complex of a branching structure, and their nodes will have more spacing and their grow tips will be more bunched... They look lanky vs their shorter squater female counter part.
Red stem vs white stem on brand new seedlings... I can almost swear by this, but, I wouldn't bet the ranch.. LOL! Red stems seem to be male in MANY cases.. IMO!
I've also seen plants that fit all the criteria for being female, and end up being cursed males..