The terminology you are looking for is Temporary Flight Restriction:
TEMPORARY FLIGHT RESTRICTION (TFR)- A TFR is a regulatory action issued by the FAA via the U.S. NOTAM System, under the authority of United States Code, Title 49. TFRs are issued within the sovereign airspace of the United States and its territories to restrict certain aircraft from operating within a defined area on a temporary basis to protect persons or property in the air or on the ground.
The "Stadium TFR" is a singular NOTAM, currently FDC NOTAM 0/0367, which creates a TFR with dimensions "3 NMR OF A QUALIFYING STADIUM OR OTHER SPORTING VENUE HOSTING A QUALIFYING EVENT UP TO AND INCLUDING 3000FT AGL ... ONE HOUR BEFORE THE SCHEDULED START UNTIL ONE HOUR AFTER THE END OF A QUALIFYING EVENT." It is entirely the pilot's responsibility to know which stadiums and sporting events are covered (there is a definition farther along the NOTAM) and what the exact start and end times are. As you say, stadiums should be marked on VFR charts as a red diamond, but the lack of such marking would (I am sure) not be an acceptable defense if you were found to have violated the TFR.
You can google the term "Stadium TFR" to find articles and forum posts discussing it. Note also that you are allowed to fly within the TFR if you meet certain specified requirements, such as if you have an operational reason to be there—as one example, many stadium TFRs encroach on the final approaches to Class B airports, and you can be sure that the airlines don't stop all their flights for those four hours.
Currently active TFRs can be viewed in graphical form at the FAA's website and at third-party sites like Skyvector.com. Skyvector will display active TFRs in red and upcoming TFRs in orange; I'm not sure about the FAA's site. I believe electronic flight bags like ForeFlight and Garmin Pilot also have this capability.
Note that the sites are probably all displaying the end times based on the scheduled end time, while the ending of the TFR is based on the actual end time. Or perhaps the sites are smart enough to scrape MLB.com's API to determine the true end time, I don't know.