Alzheimer's. There is no specific test today that confirms you have alzheimer's disease. Your doctor will make a judgement about whether alzheimer's is the most likely cause of your symptoms based on the information you provide and testing like mental status testing, neuropsychological testing, brain imaging, physical exam and lab blood work to rule out other causes of memory loss and symptoms.
Brain microscopy. Right now alzeimer's can be definitly diagnosed only after death by examining the brain under the microscope. One day there may be scans that will work. Meanwhile, one can diagnose dementia based on neuropsychological memory tests, slowing on eeg, and delay in cognitive evoked potentials, look for reversible causes using blood work and mri, do genetic testing for risk of alzheimer's.