With mixed breeds its sometimes hard to tell. Shorthair is dominate over longhair tho. I've had some kittens with "fluffy" fur as kittens and when 3-4 months old - they were shorthair. And I've had a short hair kitten that wound up with semi-long hair but you couldn't tell it at a young age.
Even if both parents are shorthair, its possible to get a longhair if both parents are carrying the LH recessive gene.