It is important to know the sex of the marijuana plants when growing using hydroponics and there is a way to tell. The male marijuana plant will produce balls, rather than the buds the female plant produces.
The reason that it is important to know the difference is that many growers move the male marijuana plant into another room to keep it from pollinating the female plants. This means there would not be the buds and amount of THC that growers spend their time, money and energy on for the amount of pot they get from a marijuana hydro crop.
The male plant will show its sex earlier than the female and they are often not as full as the female plant. The most important sign is the balls that will form and this is the sign that the male needs to be moved away from the female plants.