My Dad Just Turned 84. He's Getting More Forgetful And

My Dad just turned 84. He's getting more forgetful and sometimes confused and frustrated. He is fine physically other than not being able to walk. He just had a complete physical including cat scan and checked for any type of infections and all tests results came back ok. He lives in an assisted living place and can not walk for almost a year now. He uses a wheel chair. What are the symptoms of dementia and/or alszheimers? How do we get a reliable diagnoses?