Neuropsychological evaluation (not psychological...NEUROpsychological) can identify dementia and it can document numeric levels for many types of cognitive functioning. Such documentation or establishing a "base line" helps determine if the dementia is changing and at what rate. However: there are numerous causes of dementia. Some are manageable. Some are reversable.

Neuropsychological testing is not particularly useful in specifying the cause. Clinical history and examination by a Neurologist can be more helpful in specifying a cause.

Expect to pay close to $2,000 easily for Neuropsychological testing. So it isn't something to be done on a whim.