I've picked out the males out of a species tank just by getting the most dominant lab in the tank with more pronounced black on its fins as well as eggspots, males will also often show a little black line running vertical through their eyes, this however is not a 100%, I've seen some females with similar traits (guess I've just been lucky).
Though if you are doing it blind (without venting them or if they are smaller than 2", when you can't really vent them to be certain yet), just try to pick out the most dominant and then the lesser or least dominant labs that show less black, this way you've got nothing to lose, because either way you'd be thinning them out at maturity. Also try to pick out the most yellow ones that don't show any barring at all.
Though if you are doing it blind (without venting them or if they are smaller than 2", when you can't really vent them to be certain yet), just try to pick out the most dominant and then the lesser or least dominant labs that show less black, this way you've got nothing to lose, because either way you'd be thinning them out at maturity. Also try to pick out the most yellow ones that don't show any barring at all.