The person whose interests are going to be assumed by the Agent is called the Principal. The Principal must be mentally competent when the document is signed in order for the Power of Attorney to be legally binding. If there is any doubt about the mental competency of the Principal, a physician's must be consulted. If the physician determines that the Principal understands the document and the powers that are being assigned to the Agent, then the Principal is determined to be mentally competent.
It is generally a good idea to have your Power of Attorney notarized by a Notary Public. Notarization reinforces the validity of the document and makes it harder to contest in court. Health Care Powers of Attorney must be signed by two witnesses in order to be valid.