If the chicks at your feed store are separated into pullet and cockerel bins (my fav store labels them "hens" and "roosters"), then I would go for the most active chicks from the pullet bins. I also always chose those older than the others, more feathered out with wing tips. If some were left over from the previous week's delivery, I would choose those. They have already survived a week, which means past the critical first few days.