Sweet restaurant oranges

How do Chinese restaurants always get their dessert oranges right? I've never once had a sour orange. Do they slice up the orange normally but leave out a little slice so someone in the kitchen can eat it to make sure the whole orange is sweet? Is there an orange taster in the kitchen? Or do they have a secret contract with supermarkets to always get the sweet ones? Is there some kind of information that farmers, supermarkets, and Chinese restaurants are holding out on us on how to tell whether or not an orange is going to be sweet?