covers the power up sequence. Will also mean you have at least the start of a binary too.
As far as determining what is code... few surefire ways and they could scatter code across banks.
You don't have multiple modes
Some debugging emulators will also be able to log executed data, some even doing really fun http://fceux.com/web/help/CodeDataLogger.html
where it will note when something was executed (say you want to find a jump routine, as in jump in a platformer, get to a point in the game where you could jump, start the logger, idle, walk about... basically everything but jump, now it has a baseline of executed routines, do a jump and that shiny new code section not seen before is presumably what you want).
You can do a full playthrough of a game to get an idea for the locations of the code. Not so bad on the GB/GBC where games are normally pretty straight shot from start to finish, compared to later systems where you can teleport anywhere, load any level, maybe have some minigames...