PWS is a rare disorder, and not everyone with an over-eating problem will have PWS.
Read the article on Characteristics of PWS carefully. If you can check most of the symptons mentioned there, it may well be that you, your child, or someone you know, has PWS. If not, there are other organisations, who may be able to help (see the list at the end for links)
Our sister organisation, PWSA (USA) has published a checklist of diagnostic criteria which could help you decide whether or not the person has PWS.
If you still think PWS is possible, go to your doctor and ask for a referral for genetic testing. Take information from this site and the diagnostic criteria with you.
SWAN (Syndromes Without A Name)
National Centre for Eating Disorders
National Eating Disorders Association
Contact a Family - a UK charity for families with disabled children, offering information on specific conditions and rare disorders