There is no definitive test to tell you if someone is suffering from dementia. There are some cognitive tests that can be administered but as the National Center for Biotechnology Information acknowledges, many doctors hesitate to conduct them or even broach the issue with relatives of patients who may appear demented because of the distress it can cause. There is thus a major element of guesswork in making a diagnosis.
Still, there are things individuals can do to anticipate and plan for the possibility that someone could develop one of the various brain diseases that fall under the dementia umbrella. Considering the high cost of providing the care that's required and the fact that dementia leaves the stricken individual unable to make decisions for him or herself, it becomes clear that setting up the legal framework to support long-term care matters. Don't expect that Medicare will be enough to cover the tab.