There are some early sexing methods that aren't 100% accurate, but might help a little. If all of your seeds were started at the same time, then you might notice som plants are taller than others. Taller ones tend to be male, smaller plants tend to be female. But that's not 100%
Watch your calyx development...Other than that, the best way I can suggest is forced-flowering. You can take a clipping from each plant (make sure you keep track of which comes from which plant) put them in a growing medium (pretty much like taking clones) and put them on 12/12. Force them to flower, and then once they show signs of sex, you'll know which o your plants is male, and which is female.
First pic is preflower female, second is preflower male, third is another side by side shot.