History and tests. Taking a thorough history of functioning, changes in abilities and doing cognitive screens. It is also important to rule out other issues that could cause similar symptoms. Areas to be tested inlude attention, memory, understanding and using language, generating information, problem solving, visual spatial skills.
Few do. It is a clinical diagnosis. Accuracy can be improved with neuropsych testing, CSF levels of amyloidB42/tau/hyperphos-Tau, or FDG-PET / radio-ligand PET (FDA approved but medicare wont pay).
Future tests. Currently there is no blood test for Alzheimer's disease. In the research setting, physicians and scientists have developed tests that can diagnose Alzheimer's disease using cerebrospinal fluid. There are also experimental radiologic studies that can detect early Alzheimer's changes. Although these tests are experimental today, they should become available in the near future.