As in the human world there is a big difference between a male and female marijuana plant. As you can see from these pics, the male plants pre-flower looks like balls and the female plant has hairs.
This is a male plant - note the ball-like nodes on the stem.
This is a female plant. The hairs can be seen on the stems.
It is important to kill all males as soon as you can identify them (unless you are using them to breed).
More pics to come in a few days.