If you use a low abrasive metal polish and a light amount of pressure and there is no change in the color of the brass, that is one indication that it is coated with lacquer. Lacquer can be removed with a commercial paint/lacquer remover available at any home center. Acetone also works but may take a little longer. If a part is lacquered, you won't be able to "polish" it because the brass surface is sealed. The best you can do is clean off any accumulated dirt. Simichrome, Maas or similar metal polish for brass will only work on the brass surface. If the lacquer finish is in poor condition, remove it, then polish the brass and seal with a coat of wax (assuming you don't want to put a new coat of lacquer on).
Stan