take a clone when you can and put it in to flowering, and it should show sex within a couple of weeks. Sometimes I'll number my plants, and then the clones I take off of each plant. I'll flower the clones, so I know which plants are male or female. I really only saves you a little bit of time. Either way, you're still going to have the same ratio of males to females. You can also use the clones that are female, and put them in with their mothers and take out the males and flower them all if you wanted to. Or you could decide which plant/s you wanted to keep as a mother and take multiple clones from it.