An initial clinical diagnosis of PWS can be made by checking the diagnostic criteria
This can be confirmed by genetic testing
Methylation analysis will detect more than 99% of cases, including all of the major genetic subtypes of PWS (deletion, uniparental disomy, or imprinting mutation). A FISH (fluorescent in-situ hybridization) test will identify those patients with PWS due to a deletion, but it will not identify those who have Prader-Willi syndrome by UPD (uniparental disomy) or an imprinting error.
There are a number of other syndromes with some characteristics similar to PWS. Check here for a list of these.