An individual can prove another person's mental incompetence by showing that person cannot substantially manage his finances, that he makes repeated mistakes that a mentally competent person does not make and exhibits strange and unusual behavior, notes Grossman Law Firm. Medical records and witness testimony can help prove mental incompetence.
A mentally incompetent person can be deemed senile, delusional or of unsound mind, during contested will proceedings. Clients can contest the signing of a will on the grounds that the person who signed the legal document was not mentally competent enough to understand what happened, notes the Law Office of Andrew M. Lamkin, P.C. The individual who brings a claim of mental incompetence against a will has the burden of proof in the matter, according to Borenstein, McConnell & Calpin, P.C. The challenging party often uses expert witnesses, such as physicians, to prove to the court that the person who authorized the will was mentally incompetent at the time the document was signed and notarized.
Estate planning and wills are two areas of law that utilize mental incompetence declarations. Mental incompetence, as it relates to informed consent, can affect the execution of documents such as living wills, durable power of attorney and guardianship papers. It is important for senior citizens to sign legal documents before mental incompetence due to old age occurs, notes Texas Medicine.