Cats come in three varieties: shorthair, medium-hair and longhair. (Sometimes, medium-haired cats are considered either long- or short- haired for simplification.) Adult coat-length is difficult to determine in kittens less than 6 weeks old. A few tips, such as the length of the ear tufts or the "fuzziness" of the kitten, sometimes may enable you to do so. Ear tufts are more elaborate on longhair cats and kittens; the ear tufts will start developing even on a young kitten. Once the kittens reach about 8 weeks, their coats begin the slow maturing process; kittens destined to have long hair will be much "fuzzier" than their counterparts.
Notice the ear tufts or any patches of long fur. It's difficult to tell adult coat-length in very young kittens, and even ear tufts and long patches may not be evident if the kitten is younger than weaning age. If you are choosing from a litter, you can compare the kitten's coat with its litter mates.
Observe the kitten's coat once it is weaning age (6 to 8 weeks). At this age, the coat will be developing. A kitten with a shinier, sleeker coat will be short-haired; a kitten with a fuzzier, softer, fuller fur will be longhair.
Wait and see. Determining the length of a kitten's coat is not a foolproof process. Some kittens have very fine, fuzzy fur regardless of the future length. Young kittens' fur and characteristics develop week to week, and you should know how long the coat will be by the time the kitten is about 3 months old.