Went through this with Dad a few years ago, before he passed.

If by test, you mean something objective like a blood test, I do not know of any, although I read somewhere there are efforts to develop a test like this for Alzheimer's.

But there are tests that are subjective, where the physician asks a series of questions, tests memory and so on, on the basis of which the diagnosis is made.

Also, as I mentioned in another post some time ago, a diagnosis of dementia in an older person may not mean that legal action needs to be taken for their protection, or that they can't do things like make wills/trusts and execute a DNR order. There are different definitions and standards, and consulting with a local professional is the best way to find out what can and can't be done.

Hope this helps