In most cases, a letter of administration (or a similarly named document) will be issued by the probate court that is administering the estate of the deceased.
That document will provide the authority to the Executor to conduct business on behalf of the estate, and would be accepted by most banks and other property and financial service providers.
Thus, a probate case must be initiated to obtain such a letter.
If an estate case is not opened with the probate court, there will likely be issues with processing any checks made payable to the deceased or handling any other official business on behalf of the deceased.
To open an estate case, contact the clerk of the probate court to obtain information on the process. Or if you need advice specific to handling a probate case, Standard Legal always recommends you consult an attorney; you can find a lawyer for FREE at Standard Legal’s Attorney Find page.