The best indicators can normally be seen whilst they're still with their family group, if you get the opportunity. There will always be one kitten who's more boisterous or playful than the rest, or one who is "explorer kitty" - they may be the ones who come up to you first, or are first out of the box or bed. They will normally confident and fairly assertive cats. Large and in Charge. They will probably be first to try out new things, and get into mischief more readily. They are normally sociable, and have leadership qualities but can tend towards bossiness.
There may be one who shies away or is more reluctant to come to investigate what's going on. They may be more uncertain of new experiences and take time to assimilate those new things (doesnt mean they're slow - just cautious!
There may be those kittens who dont react immediately to a new presence cos they're too busy playing.
Face shapes can be a good indicator too - and paws. My personal experience is that larger rounder face shapes and paws tend towards more laid back characters, whereas more pointed faces and larger ears can be indicative of oriental ancestry, and these can be quite enquiring, active, eager to learn cats, who can have more complex qualities to their natures, but that can often be sensitive to external stimulus and maybe not as receptive to potential stress factors (like small children, loud noises etc).
There are always exceptions to every rule, but these aren't so much rules as guidelines - generally they will serve as pathways to help assess a kitten's nature when you first meet them.
