Sexing Your Cockatiel

Someone that is in a parrot group said that you can sex a cockatiel at the age of 6 months. Stating that females sing, boys whistle and something about how bright their cheek colors are.
My response:
This is not true. You cannot properly sex a cockatiel by appearance or vocalization. The only way to know for sure is to get a DNA test. A vet, a breeder and Google will tell you that.
They replied:
It has been going well for me so far.
Sure, you have a 50/50 chance of getting it right by guessing but it is always best to get a DNA test especially if you want to breed so that you can provide the proper diet. It's the same even if you don't want to breed.
I paid $25 for Lord Luka to be properly sexed. It is affordable. But I'm sure the vet might charge more for bigger birds, it's just what they do. It is worth it though.