Unfortunately, if you want to breed to the SOP, it takes many months before you can really tell quality. Some things you can cull for early on, like insufficient leg feathering, sprigs on the comb sometimes show up in a few months on the males, incorrect breast color (too much) can show up early on in the males, deformities in their legs (knock-kneed). But in general, best to kept them until at least after their first adult molt at 5-6 months, but even then, some things don't show themselves until the birds are close to a year old or more. Their tail set can continue to change, for instance, and one with good angle on their tail when young can end up squirrel tail when mature. You cannot rush these birds. IMO, that is one reason it is difficult to get good ones. People breed them young, before they have a chance to really mature and show their faults. If everybody could determine quality at a young age, there would not be any challenge in breeding quality birds. But such is not the case. The successful breeders raise up many, many birds and only keep a few. It can be difficult, depending on your situation, to keep a lot of males once those hormones start kicking in and they start squabbling with each other.
I got my first BCM chicks in Dec 2012 and made my first culls about 4 months for males with too much color on their chest. I ended up culling everyone of them by 9 months for different reasons, after too much color was not enough color and very poor body type along with incorrect hackle color.I agree with desertmarcy. Our first culls are around 3 months and we are looking for the really obvious DQ. At 4-5 months it really is observation, we cull anyone who doesn't look as good as the others- usually to good homes. We ultimately cull to our top 5 roos at 5-6 months. All the others we move along, just for space reasons. Again we compare them against each other, breeding records, dark egg hatches, parents, needed traits, etc. We will only end up keeping our very best 2 roos with a combinations of parents, dark egg hatches, looks, and what we need to pair our pullets/ hens with being the deciding factors. We first see our best judge of adulthood traits at 7-8 months. We have very few surprises after that. BCM are tough because they develop so slowly. You have to plan to be in it for the long haul and not second guess yourself if you decide to move younger along ones due to looks, space, etc.
If you just have a few to select from, try to keep your best 3 for as long as possible. Look for hackle color, body shape & size (knowing they will fill out more)- long/ short necks, tail/ wing proportions, size of feet/ legs are a good indication how big they will get (like a puppy growing into his paws), feather color, etc. Do take into account the darkness of the egg your roo hatched from as the dark color of eggs is further improved with males that have hatched from dark eggs- they will pass along this gene and it can be the difference between a 6, 7, or 8 color in next years pullets.
Hope that helps!
this year I am growing out my first crop of my own breeding. I am culling heavy and culling early. My first cull was at 2-3 weeks for lack of leg feathering. My next culling, on birds 8-12 weeks old, which will get rid of most of the males I have will be for too much color on the chest and poor hackle color, along with signs of poor body type like pinched tails, too long of a tail, squirrel tails. That will cut me down to about 4 babies, plus the two older ones I showed this morning. By fall I should be down to 2 or 3, along with my older cock. After some test hatches I'll cull down to the best one bird from 2014 to go forward with along with the older cock bird.
I hatched from 4 pullets this year. After looking at the results I will pick only the one or two hens that gave me the most good looking chicks to breed from this fall.
For what it's worth I've been reviewing the pen of early culled birds that lacked leg feathering. Everyone one of them also has other issues showing up now, like poor hackle color, pinched tails, long tail, all chest & no butt. So I feel like I made a good decision there.