Dear Dr. will,
Greetings and welcome to justanswer,
The Alzheimers disease can be diagnosed by the neuropsychological examination and various test like blood test, thyroid profile, pet scan of brain to rule out the other causes of dementia.
The exact diagnosis is made by the microscopic examination of the brain and is possible only after death.
The neurologist can interpret the test and diagnose such disease based on the symptoms and results.
here is the link for more information
test fo alzheimers disease
Please do ask me if you have any more question