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.