Enable the WDT, and possibly BOR,
then have code at the start of the program, that if, when it starts up, it detects that it was a WDT reset or BOR (anything really except for normal powerup), if it does see the flag, then light up a LED to let you know it locked and was restarted.
Although, I am more used to PICS, so I do not know what functionality like that the Arduino has.