As an Executor, how do you make sure a handwritten will is authentic?

If a will is written in someones own handwriting that is called a holographic will. In Pennsylvania unlike many other states it is valid. You will need two witnesses that can identify the signature. You will have to have to have a copy fair which is a typed transcript of the will with what you interpret as the authors intentions. You will have to make an appointment and go to the Registrar of Wills who will take this information and determine the wills validity. A holographic will is not what you want if you are planning your estate. A holographic will is more work and the state will interpret it rather than you.