The best way I have found out is to
1 wait till you have 4 branches
2 start clones off them mark all plants
3 after roots have started switch the space to 12/12
your primary plants have grown for 4 weeks
at first signs of males trash them ,,, note first signs (they are numbered )
4 replace the known males with females in the on you primary system
you will loose some groth with them but will harvest all females
The gain is you can test your females sooner and cull any ,and find out which, its going to be like. (you might be supprised there may be a differance)
Remember to clone the good plants before switching the primary plants to bloom
ADVANTAGE
you get to test the strain, and have all females, clones for the next run.