A power of attorney is a powerful legal tool because it allows an attorney-in-fact to perform binding legal acts, such as signing contracts, on behalf of the principal. The principal has no need to resort to challenging the attorney-in-fact's authority unless he is seeking redress for damages arising from acts already performed, because the principal is entitled to revoke a power of attorney at any time as long as he is competent (mentally sound and able to communicate). A third party, however, may have an interest in challenging a power of attorney, especially if the principal has become incompetent due to injury or illness.
Examine the power-of-attorney form to determine if it meets state formal requirements for validity. Although state laws differ, most states require a power of attorney to be witnessed or notarized. Some states, such as Pennsylvania, require the inclusion of certain statutory language. Other states, such as New York, require a separate statutory form to allow the attorney-in-fact to make gifts out of the principal's assets. Failure to comply with these requirements will render a power of attorney void.
Investigate the principal's financial, legal and medical records to determine if the attorney-in-fact has been abusing his authority. An example of an abuse would be if the attorney-in-fact authorized an attending physician to perform an act that is contrary to the principal's wishes as expressed in a living will. If you do not have access to this information and the attorney-in-fact refuses to cooperate with you, you may have to wait until you file a petition with a probate court to obtain this information.
File a petition with the probate court with jurisdiction over the principal's residence, alleging abuses of the agent's authority that you have been able to uncover and seeking revocation of the power of attorney. You must state your relationship to the principal and explain why you have an interest in the case. You must also allege specific facts rather than general allegations. You don't have to be able to prove the facts you allege at the time you submit the petition, but you do have to demonstrate a rational basis for filing it.
File a discovery motion with the probate court to compel the attorney-in-fact and any other relevant parties (such as an attending physician) to disclose any information, such as financial or medical records, that would provide evidence to support your petition. Submit any evidence you uncover to the probate court.
Petition the probate court for legal guardianship over the principal if the court denies your petition. In many states, an attorney-in-fact must obtain the guardian's permission to perform legal acts on behalf of the principal. You cannot be appointed legal guardian over an adult principal unless he is incompetent.