You can generally arrange for testing for dyslexia with an educational psychologist.
If your child is in public school and having difficulties, you are legally entitled in the U.S. to request evaluation through the school district. You can start the process by writing a letter to the school principal requesting evaluation for learning disabilities.
Please keep in mind that dyslexia is not a disease or an identifiable physical condition. Rather, it is a learning style that usually can be assessed through a profile that shows whether your child has a typical pattern of strengths and weaknesses, coupled with assessment to rule out other possible causes of symptoms, such as vision or hearing problems. Formal assessment is usually necessary before your child can qualify for special services from his school, but it is not a prerequisite for getting help on your own for specific problems that you are aware of, such as reading difficulties.
Most Davis Dyslexia Correction® providers do not provide [full testing and evaluation services], but do provide a screening test geared to determining whether our program will help the student. Davis Certification and Licensing by itself does not qualify a person to provide full diagnostic services, but some Davis providers are also qualified as medical doctors or psychologists, or hold other advanced degrees, or work in affiliation with individuals who have such qualifications. Always ask what qualifications a person has for the services they offer.
Be aware that many individuals who claim to offer diagnostic testing services do not have the sort of educational credentials that would be seen as acceptable by schools or other agencies. There are some fee-based services available online that may provide an informative report, but would not be accepted as providing an [official diagnosis or documentation of dyslexia] or any other learning disability.