Well as far as I know the only way to determine the sex of your plant/s is when the plant is budding, if you see seeds attached to the plant then it is male, if not then it is female.

Also, I doubt you can tell the sex of the plant when it is in it's early stages of growing, when it starts to bud you should keep an eye out for seeds. If you have more then 1 plant growing remove all male plants from the females (this will lessen the chances of your females turning into males). From what I have heard male plants urinate on the female plants (if grown next to each other) which will convert the females into males, female plants are best for smoking as they have no seeds.