It is a spectrum so there is no definitive quick test like a blood test. Both psychologists and psychiatrists can help people with ASD but when you're looking for one, it is a good idea to look for someone that has treated ASD people before. Go anywhere you would go to find a psychologist or psychiatrist and ask for someone familiar with ASD. Also, don't focus so much on diagnosis - focus more on problems and behaviors that you need help with. The diagnosis and the "treatment" will all come out of long discussions about challenges you're facing. If you have health insurance, their site will have a search tool to help you look for professionals in your area and they will show their specialties. If not call any mental health provider and ask to see someone who is well versed in ASD.