Most conditions are diagnosed by health professionals. The article "How to access health services" explains how to access different health services - what might prompt a referral, who can make the referral and what the service could offer.
The disability charity Contact have useful information about getting a diagnosis.
Families who are waiting for an assessment for autism or another neurodevelopmental condition can find information and advice on the Sheffield Children's Hospital website. There is also a guide that explains the autism assessment process and an Autism Mythbuster page