Shutting off both engines would display "ENG SHUTDOWN" in yellow text (caution) on the EICAS. If only one engine was shut down, it would say "ENG SHUTDOWN L" or "ENG SHUTDOWN R".
Any of these would trigger an unmistakable audible "BLEEP BLEEP BLEEP" to draw your attention to the screen so that you could see what the caution was. These messages are right next to the engine N1 indications anyway, so it would be immediately obvious that one or more of the engines was spooling down.
Shutting off both engines would display "ENG SHUTDOWN" in yellow text (caution) on the EICAS. If only one engine was shut down, it would say "ENG SHUTDOWN L" or "ENG SHUTDOWN R".
Any of these would trigger an unmistakable audible "BLEEP BLEEP BLEEP" to draw your attention to the screen so that you could see what the caution was. These messages are right next to the engine N1 indications anyway, so it would be immediately obvious that one or more of the engines was spooling down.