It's always better to know the sex of your plants ahead of time and only put out females. It's expensive and a waste of time to, say, put out 20 plants of unknown sex and take care of them for 3 months only to have to pull half of them anyway.
The best way to do it, I think, is to start a bunch of seeds early, and take a clone off each plant. Keep track of which clones came from which plants and put the clones under 12/12 while still vegging the original plants. Once you know the sex of the clones, you know the sex of the originals and they can be your mothers. If you start early enough, you can grow out each female clone and only put out clones of from the corresponding plants that you like the best.
Either that or use feminized seeds. I don't have the room right now to do what I just suggested, so I always used feminized seeds. A lot of people don't like fem seeds, but I've grown around 50 plants over the past few years from feminized seeds and have yet to have one hermie.