Is the plant in flower or veg light? (12/12 or 18/6 or 24/0)
-If still in veg light you should clone it and flower the clones. if they are female the plant is female. start your clones on 12/12 flower light Immediately upon cloning. They will show signs of sex as soon as they root.
-If you dont want to wait for this, compare all the plants you have that came from the same seeds. Example, If you grew 10 plants from BigBud seeds from a reputable seedbank, that were all given the same growing conditions (light, fert, soil, etc), compare them. You will notice that some look the same as others. Some tall, some short, some bushy, some leggy, some with 2 inches between branch internodes, some with 1/2 in between internodes. (see picture).
-If they all came from the same parent, and they were all grown the same, then the differences you see will be caused by sex.
-Which traits are which sex is the tricky part. This depends on the strain. I used to think that males always were taller and leggyer and females were short and bushy. Used to think that females would get pre-flowering hairs at about 6 weeks old in Veg light. -Boy Was I Wrong!!
These are good rules of thumb but different strains can surprise you.
" Surprise, im a boy, and now you have seeds!"
talk to the bank you bought the seeds from or the original makers of the strain. ask them what Vegetative characteristics indicate male and female plants. Once you know this, or clone for sex yourself, you can use the comparison method for future sexing from seed but only if you have a nice group of plants for comparison that were all grown together.