Alright, I feel the need to chime in to correct the misinformation here, I have a feeling this was just a random act of googling, so since it was brought up...
Actually, that's the very nature of Prader Willi syndrome. Individuals are not "genetically fat", they are born with a series of malformations, one of which is a moderate to severe mental handicap that prevents them from having any understanding or ability to control their eating habits. They are ALWAYS hungry and will never stop eating. They cannot have ready access to any food and will constantly ask for it. They will eat garbage, literal garbage, so garbage cans have to be locked and barred down as well as kitchen cabinets.
Their obesity is caused by overeating, which might be the opposite point you were trying to make...
Yes you would know. Like Down syndrome - there is a characteristic set of malformations that are readily identifiable. The average obese guest at Disney also does not have a moderate-severe mental handicap (no... don't make me go there!)