You can tell by disconnecting the display and booting the game. Press to get into the diagnostic (presumably makes a sound or something). Or you can go into the menu and try and start the game.
In almost all cases Diagnostics are part of the operating system, which may or may not be tightly integrated into the 'game rom'. Some systems (bally and williams in the early 80s) have discrete roms for each. Later on they just compiled the gamerom and OS rom together as one. Likely this is what stern is doing as well. It doesn't make sense to have a separate display processing subsystem that would then control anything on the main system board diagnostic wise.
Dunkirk, MD
