First off, ideally, to prevent data loss, you should use more than one replica. For better throughput, use more than one partition.
When you describe the topic, it will tell you the leaders for each partition. That will give the broker ID. You will need to make a note of which ID's belong to which machines as well as the data location for each broker to know where the data is stored on those servers.
As for how it determines, there is a leader election algorithm within Zookeeper... probably worth reading over the Kafka documentation / Wiki if you are really curious about that.