My mom was diagnosed 7 years ago with AD, and she saw a neuropsychologist or neuropsychiatrist who did a series of tests, as mentioned above and also interviewed my mom, well that was part of the evaluation. My mom also had an MRI and in addition, bloodwork to rule out other diseases. Her diagnosis is "probable" Alzheimer's because a definitive diagnosis cannot be made until autopsy, when the brain is looked at for "placques and tangles"...there is a wealth of information on the Internet that you might want to Google.
Just as an FYI, two weeks ago, she began having TIA's, "mini strokes" and after 3 days in the hospital and every imaginable test, we learned that they were been caused my medication. She was taking Sudafed, which can cause strokes in people with high blood pressure. You need to find out what medicaitons uour dad is taking, including over the counter drugs and supplements.
Best of luck,
Fibromyalgia, Osteoarthritis, scoliosis, back problems, hypothyroidism.