If an adult was suspicious that they have an autism spectrum disorder; what's the way to get diagnosed?

See a psychologist.: They will do a complete evaluation, including psycholological testing, which will almost always give the correct answer.

Also: They can apply for ssi, which will provide an assessment, or your state's vocational rehabilitation agency, which will assess current functioning & needs. Of course, complete neuropsychological and psychiatric evaluations are optimal, if funding is not an issue.