I'm assuming you're asking because you want some document or device, such as a power of attorney, advance directive for healthcare or conservatorship giving another person legal power to deal
In all such cases, state laws require that the decision must be made in writing by one or two doctors or other healthcare professionals. This legal requirement sometimes seems a little offbase, since a doctor may have no idea whether your parent is equipped to deal with making financial decisions. But it's almost always done that way.
If you are convinced that one of your parents has become incompetent, you may want to gather some tangible evidence, such as long unpaid bills or documentation of dangerous living conditions, to help convince his or her attending or primary physician that some type of legal supervision is needed.
with your parent’s care or finances or both.