If your cat is not neutered the easiest way is to feel down there and see if you feel 2 little fuzzy peanuts. My husband claims this is 'rude' but honestly I find it the easiest way to sex older kittens.

Honestly with very furry and extremely young kittens it can be difficult to tell. If the cat was seen by the vet at a very young age and has alot of fur it wouldn't surprise me if the vet had trouble sexing the kitten.

Now if the kitten is older and not particularly furry then any vet should be able to correctly sex the kitten barring any weird things like undecended testicals.