My husband may have undiagnosed autism. How do we go about getting an evaluation? Is this psychological or neurological?

Excellent question. Start w/family doc. He/she will probably have appropriate referrals for you. You may see a neurologist to rule out medical conditions. Ultimately a clinical psychologist or neuropsychologist will be helpful. Peace and good health.
In Autistic Spectrum . Disorder, like all Neurodevelopmental Disorders, manifestations of atypical brain development occur before age 18. However, many parents recognize their own signs of ASD 1, previously called Asperger Syndrome or High-Functioning Autism, only after their children are diagnosed with ASD. Neuropsychologists, psychologists, psychiatrists & neurologists - often as a team - evaluate & manage adult ASD .