Normally, you'll have to raise them up before they show their adult characteristics enough to make an educated choice.
As you become more experienced with the breed, some flaws will become evident even in chicks so you may be able to cull earlier in the future. Meaning that, flaws and disqualifications observed in adults, with experience, you'll be able to detect those characteristics in chicks.
Just because they come from good stock, doesn't mean all the chicks will be excellent examples of the breed. It really doesn't work that way.
Out of every 20 cockerels I hatch, there are usually only one or two that are good enough examples of the breed to retain. I can probably determine 3 or 4 chicks I won't be keeping and then as they grow, I'll identify others due to color, comb, earlobes, conformation, etc..
! I hope that you enjoy this site as much as the rest of us do!! i Have learned SOOOO much from this site, and I hope that you do too! 
I did 2 sets this year because I did something wrong with the first set and had a poor hatch rate. That is when I discovered my rooster wasn’t all that great. It was the first time I used him. His chicks were hard to sex sometimes. He was a bit wacky anywho. In the past I borrowed a friend’s rooster to make more girls and I had never had a problem sexing chicks. So when I decided I needed to cull my rooster I asked yet another friend with legbars if I could hatch his eggs so my rooster would not be related to my hens. That is when I found out his chickens came directly from the best of greenfire farms, so that is pretty cool. Now from his eggs there are 7 roosters to choose from and I need to pick a couple. I was hoping it was going to be an educated choice. I know there are never guarantees but I am starting with a good set of chicks. 
