My son was tested for PWS at around 3 weeks of age. He would not feed at all so that was the first red flag that something was wrong. They tested for a number of different things, I think. PWS diagnoses is done through genetic testing. There are a couple of "types" and different tests to be done. the DNA methylation will rule out all types of PWS with 99% accuracy.
for info. on this and other diagnosing criteria.
For males, there is a very high incidence of undescended testicles - over 95%. For a person who is older and not yet diagnosed, they would likely be short in stature and have incomplete puberty, and often small penis/testes. Another common trait is lack of vomiting. But please note that PWS is a SPECTRUM disorder and not all features apply to each person.
Many families with PWS do lock up the fridge/cabinets and find their children function better with this food security. Sort of an out of sight/out of mind thing. If the person with PWS knows that there is no way they can get to any food then they don't obsess over ways to sneak and get to it.
There are also "PWS-like" cases where a person does not genetically have PWS but can have all the classic symptoms. This is more rare but as we are learning more about genetics and various micro-chromosonal issues I think we will find new "syndromes" more and more through micro-arrray chromosonal testing.
The most important reason to do testing, IMO, is that there are specific medical issues to PWS and other disorders that can help save the child's life if you are aware of them. Also there are treatments for certain aspects - like with PWS growth hormone treatment has really improved the lives of many people. Our children who are on GH have improved muscle tone, higher IQ's and more energy. Feel free to ask any more questions about PWS!