Did you know that queen bees "quack"? (It's officially called trumpeting). They do this as they are introducing themselves to the hive, or while there are other queens that they are going to battle for dominance of the hive. It basically announces, "Here is a queen bee!" to all the workers and drones. You can actually hear it quite readily from about 10 feet or so from the hive.
If they can get the hive to accept them as THE queen, they won't even have to battle the other queens, the workers will actually "bake" the other queens to death by crowding around them until they create heat in the center at over 125 degrees...
This is also what would happen if you just opened a hive and dropped in a queen without killing the other one first...
You can purchase queens and have them mailed to you if you need to "requeen" a hive quickly and don't want to wait for your hive to create a new queen...