How can you tell from girl and boy kittens?

my vet told me to look for the "2 holes" under the tail. If you can put a finger width between the two, then it should be a male. If close together, is is a female. He said no scientific evidence to that but in his practice of 30 years, it usually worked out that way. I figured out the gender of my 5 kittens at age 2 weeks and I was right with every one of them!