If the provider believes that the service is not or may not be covered by Medicare, they are required to issue you and Advance Beneficiary Notice of Noncoverage (ABN) advising you that you may be financially responsible should Medicare deny payment. If you are not issued an ABN, the provider is not allowed to bill you should Medicare not pay. See link below for more detail:
Also to clarify, Plan G is to cover out of pocket expenses for traditional Medicare and does not have a separate list of covered benefits or standards.