The legal procedure for declaring a person incompetent consists of three steps: (1) a motion for a competency hearing, (2) a psychiatric or psychological evaluation, and (3) a competency hearing. Probate courts usually handle competency proceedings, which guarantee the allegedly incompetent person Due Process of Law.
The short answer is that it depends on state law, as each state is slightly different. In Colorado, no doctor has that power, as only a Court with jurisdiction can declare someone legally incompetent (the Court will usually rely on one or two doctor evaluations as part of the process). However, many legal documents, such as trusts...
Instead, any interested person may get involved. For example, if a government agency petitioned a court to declare an elderly woman incompetent, the woman’s children could come before the court to argue that their mother was competent, even if the children did not receive formal notice of the hearing.
If you decide to try and have someone declared mentally incompetent, then it needs to be for the right reasons and your intentions need to be in the best interests of the person in question. Jim Treebold is a North Carolina based writer.