when to sex

3 or 4 ounces of terribly seeded weed will yield about 1/2 oz smokable. (rough guess) Tremendous decrease in yield.

You can usually let the males go till you are darned sure they are plenty of male parts showing, but keeping them around while trying to harvest the 6 or 8 bonghits you'll end up with makes it wise to take the shears to him.
Have also had females I put into flower too early show initial balls, but went immediatelly to female the rest of the grow. So now I give 'em an extra few days to repent, but toss 'em before they can do any damage.