Examine the power of attorney authorization form against state law to find grounds to challenge its validity. Although state laws vary, in all states, the principal must sign the power of attorney. Someone else must also authenticate the principal's signature. In some states, the principal must sign in the presence of a notary public, who must affix his seal to the document. Other states require the presence of two witnesses, who must sign the document. Some states, such as New York and Pennsylvania, require the document to include certain statutory language.
Identify any abuses that you suspect the agent has committed. An agent must act strictly in the best interests of the principal and, in most states, may not profit even incidentally from the performance of his duties except for a reasonable fee authorized by the principal. He must also manage the principal's affairs with reasonable care.
File a petition with the branch of the state district court with jurisdiction over the principal's residence, seeking judicial revocation of the power of attorney and alleging your grounds for seeking revocation such as failure to notarize, or abuse of authority by the agent. Alternatively, you may seek an appointment as the principal's legal guardian, so that the agent cannot perform any acts on behalf of the principal without your consent.
Submit a discovery request to the agent's attorney and any other concerned party, seeking documentary evidence to support your grounds for revocation. You may, for example, ask the agent to produce records of financial transactions related to the principal's property, or ask a hospital for copies of the principal's medical records. If the recipient of a discovery request refuses to cooperate, you may ask the court to issue an order to produce the records you seek.
Subpoena any witnesses who might provide favorable testimony -- a bank officer, for example, or a doctor.
Attend the hearing and present your case. If you win, the court will issue an order revoking the power of attorney or appointing you as the principal's guardian, depending on the content of your petition.