You will be able to easily identify the main breaker as being the two-pole breaker that is at the top of your service panel. The main circuit breaker is switched on and off by either flipping its switch left to right or up and down depending on the type of breaker.
Main breakers connect to the feeder wire coming into the service panel. It is connected to 240 volts which is used to power your house. The breaker, when off, does not power the hot bus bar. Without power to the hot bus bar, none of the other branch circuits will have power. By controlling power to the hot bus bar, main breakers control power to the whole house.