Example from google: Oil Discharge: The USCG oil discharge placard that you need to have must be a certain size and placed in the proper location. The placard must be 5 inches by 8 inches in size, and it must be placed in your machinery space in areas where it can be easily seen by everyone. It may also be placed at the bilge or the ballast pump control station. The language of the sign must be in a language that is understood by the crew aboard your vessel, so you may need a sign in multiple languages in order to comply with the regulations.
I have twin outboards. I do not have a generator. I do have an installed head and holding tank. I placed all of my placards on the inside of the console door. I was boarded by the USCG last year and they didn't object to the location of the placards.
I suspect for a boat like mine (newer CC with outboards and super clean) the USCG, Harbor Master, Environmental Police, etc. would be mostly lenient and their expectations for the placards. I believe this to be especially true, if everything else (safety equipment, no booze on board, etc.) is in good order.
I suspect for larger inboard boats they would be more strict.