The only way to see these papers and test their validity is with a legal action (lawsuit). You would have to bring a legal action in family court to have yourself declared your father's guardian. In the lawsuit, you would also name and provide notice to your aunt, so that she has the opportunity to come and show whatever documentation she has of having a medical power of attorney/health care proxy, a "regular" power of attorney, or other forms providing her authority to act on your father's behalf in this situation. She would have to provide such papers in the case to oppose your application to be his guardian, and you would also have the opportunity to challenge such papers if she has them but there appear to be irregularities in them. This type of suit is substantially more procedurally difficult and complicated than, say, a small claims case: retain an attorney to help you.