I agree with Carmel, but, as someone who does TNR on a fairly large scale, I know that socializing every cat isn't something that rescues, shelters, or individual trappers necessarily have the resources for, so I'll address your question from that perspective.
Sometimes you get an inkling that a cat is socializeable upon first trapping it, but if you're committed to socializing any socializeable kitten (or cat), I would definitely advise you to wait at least a week and observe the cat's behaviour. Cats are scared when they're first trapped, and even tame cats will sometimes flip out in a trap initially, so first impressions are often not an accurate way to gauge a cat's sociability. Case in point, on Tuesday night I trapped a 7-8 month old orange tabby that I know to be one of the tamest colony cats I've ever
encountered, so tame in fact that we picked him up, upon which he began to loudly purr, and put
him in the trap... commence total freak out, complete with hissing, spitting, and trap damage from ramming his face into the bars in an effort to escape! Obviously, the fact that we approached the cat, played with him, and finally just decided to pick him up and put him in the trap with zero fuss was a bit of a giveaway that the cat we were dealing with wasn't feral; but, if we had, say, set a trap, left for 5 minutes, and come back to find this cat in the trap, thrashing around, things might have been a little less clear.
Making split-second decisions about sociability is even more problematic when it comes to feral kittens because, well, they're feral. What you're looking for in a feral kitten are indicators of potential
sociability. If a cat is too frightened by the experience of being trapped to exhibit any of these signs, it becomes really difficult to know if a cat is worth working with or not. I find that it generally takes at least a week for a cat that we would consider "borderline" to settle enough to be properly evaluated.
The positive signs that I do look for when I'm evaluating a cat are things like: Can I touch the cat? Does the cat meow? Where does the cat position itself in the trap in relation to me? Does the cat seem curious about me or what I'm doing when I'm moving around the room? Does the cat sniff, even subtly, when I put my hand near the trap? Does the cat react by sniffing, moving forward, or leaning forward when I put food in the trap? Is the cat interested in what's going on around it? etc. Body language is another good indicator, although kittens are often naturally skittish, and it's not uncommon for them to back away from people, even when they're perfectly happy to be held and cuddled. I don't worry too much about hissing when it comes to kittens either, as hissing seems to be one of the last "feral" behaviours they retain during the socialization process. I find the same thing with adult cats actually, but I don't want to be responsible for anyone getting gored.
I typically trap a day or two before a set feral spay/neuter clinic date, so it's often not possible to evaluate the cats prior to surgery. Most of the cats that I adopt out are eartipped. Does it look great? No, not really, and appearance is, of course, a factor for most potential adopters, but I try to "sell" my cats on their personalities. I figure the "right" person won't really care too much that their cat has a little bit of ear missing if they fall in love with the cat based on it's character.