It sure is hard is hard to pick! Besides what's already been mentioned (5 toes on each foot, dark skin, down covering on toes) I also look at combs, wings, and toe spacing if I can't grow them all out. Out of these the wings is the biggest criteria for me but I have to wait at least 3 weeks or so to judge how the wings are folding. Combs and toe spacing can be judged at hatch but are minor criteria (unless they happen to have single combs).
Slipped wing where the primaries fold over the secondaries is a common fault in silkies and something to breed away from. A chick that has an improperly folding wing at 3 weeks may correct in time but more often than not mine don't (it depends on the line though). The ones with correctly folding wings at 3 weeks may later get slipped wings too but are less likely to than the ones who already have slipped wings at a young age.
I wasn't able to find really good pictures of younger chicks showing slipped wings but here is a blurry picture of one of my girls that has a nice wing on one side and a slipped wing on the other. You can kind of see how the primaries on the left wing are sticking out more in this picture than the primaries on the right wing. She still has one slipped wing and one correct wing now at over 2 years old.
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Here are some chicks around 2-3 weeks where you can see one with loose/slipped wings and one with correctly folding wings.
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And here are some at probably close to 1-2 weeks where one shows a slipped wing and the other has a nicely folding wing. I try not to judge them this young too much as often my 2 week old chicks with slipped wings will correct by 3 weeks.
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You can actually kind of tell even at hatch if they might have a rougher comb or horns at the end of the comb and of course single combs are usually obvious. I prefer a smooth comb with no horns though it is a minor criteria. It's easiest to see this right after hatch when they are still wet and there isn't any fluff in the way.
If everything else is the same but one chick has really nice toe spacing between the 4th and 5th toes I'll keep the chick with the better toe spacing. Again it's a pretty minor criteria though.