The main things to watch for that are defects you don't want to breed in are side sprigs (on the comb) and split wing(missing flight feather). I judge for split wing at 6 to 8 months. I judge for sprigs as early as 8 weeks and up until they are laying age and even keep an eye past that for anything that may pop up. You want yellow legs, some mottling is ok, flesh color is ok, white/black/blue/green legs are not ok.
Watch for good personalities and pics out birds that have the ability to be good layers and also be broad enough to be worth raising for meat. Narrow bodied and pinched tails are removed from my breeders here as they won't grow enough meat to be worth butchering and pinched tails usually indicate poor layers due to the duct being narrow.
Another thing I am culling for are late bloomers. I want to be able to be fairly sure of the sex by 8 weeks and not have surprises.