Taget från The Cannabis Grow Bible

First Early Sexing Method:
If you have been growing the same strain and all the seeds at
the same time, then you may notice that some plants are taller than
others. This is a sign that the smaller plants are female and the taller
ones are male.
If you want too, you can separate your plants into two sections
in order to see how good your guesswork was when you do finally
identify sex. The other thing to know is that male plants in general
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 early date is to look at the
calyx with the aid of a very fine magnifying glass. (See last page of
this book for pre-flowering/calyx illustration)
If the calyx is raised on a small short stem then it is probably
a male. If the calyx is not raised on a small short stem then it is
probably a female.

Third Early Sexing Method.
Force-flowering a cutting is probably the best early sexing
method out there. Simply take a cutting from the plant that you want
to sex and place the cutting in a cup of water or into a cloning medium
such as rockwool. Give the cutting 12 hours light and 12 hours total
darkness. The cutting will flower and display its sex. Clones will carry
the exact same genetic make-up as the plant it came from, including
sex.

These methods are NOT 100% accurate but will help you to
understand more about the visible differences between male and
female plants. In a moment we will explain how to identify sex
properly and completely.