I stole this but it explains things pretty well:
After a few more weeks of this secondary growth your plant is looking more bushy and certainly has more node regions. It is during this time that your plant has reached sexual maturity and is ready to show sex. How long this takes depends on the strain you are growing but after the seedling stage has finished you are looking at a time period of 4 to 8 weeks vegetative growth. With Sativa strains this can take much longer.
At a certain stage towards the end of vegetative growth the plant enters its pre-flowering phase and, as a grower, you need to tailor your grow space and gardening approach to this new stage in your plants' life cycle.The next section explains how to identify the pre-flowering stage.
PRE-FLOWERING AND EARLY SEXING
Recall that during pre-flowering, plants start to exhibit their sex. As a grower, you should be hoping for as many females as possible. Pre-flowering occurs at the node regions. Towards the end of vegetative growth you need to check your plant nodes for what is called calyx development. A clone will carry the exact same genetic makeup as the plant it came from, so if you know your clone's history you will already be able to predict it's sex.
CHECKING FOR CALYX DEVELOPMENT
Choose a plant. First of all examine the node regions of the plant where the branches meet the stem. You are looking for very small pod shaped organs here at these regions.* If you don't find any here then move outwards along each branch checking each node region until you come to the tip. If you do not find calyx development then your plant has not reached its pre-flowering phase yet. You need to wait for it. Calyx development will come in time.
There are three early indicators of plant sex, but they are not 100 percent accurate.So remember, these methods can fail, but are often accurate predictors of your plants' sex.
First Early Sexing Method
If you've been growing the same strain and all the seeds were started at the same time, then you may notice that some plants are taller than others: the smaller plants tend to be female and the taller ones tend to be male. You can separate these plants into two sections in order to see how good your guesswork was when you do definitively identify sex. The other thing to note is that male plants generally start to pre-flower before females. If you have taller plants that are producing new growths before the smaller ones then the taller plants are probably male.
Second Early Sexing Method
A good way to identify plant sex at an early date is to examine the calyx* with the aid of a very fine magnifying glass. If the calyx is raised on a small, short stem then it's probably a male. If the calyx isn't raised on a small short stem then it's probably a female.
Third Early Sexing Method
'Force-flowering' is probably the best early-sexing method. To force-flower a cannabis plant, simply take a cutting and place it in a cup of water or a cloning medium, such as rockwool. Expose the cutting to 12 hours of light followed by 12 hours of total darkness. The cutting should flower and display its sex — however the plant must be mature enough to present its sex. An immature plant will not show sex because initial calyx development is not photoperiod-related. Plants normally mature around the forth week of vegetative growth because sex is not genetically determined until the third week of growth.This also applies to 'feminized seeds,' which can, and often do, turn out to be male.** If your plants are exhibiting calyx development, then this is a suitable method of determining the plant's sex.
