Simple. Newborns blood is tested before hospital discharge and again at 1-2 weeks. The test relies on increasing abnormal byproducts of protein metabolism if they do not have functioning phenylalanine hydroxylase.To pick up the abnormality, the kid needs to have eaten enough,so the second test makes sure a quickly discharged baby is not missed.It take very little blood.Older kids can also be tested.
Easy. Every newbnorn is tested for this in a state lab in the U.S.. If this was not reported at birth the person does not have pku.