As described previously about challenges involved with identifying master and slave MySQL servers, it cannot be said conclusively about which is master and which is slave.
Any repetitive one-way traffic between any two MySQL servers gives us an idea that one of them is the Master server, and the other the slave. But, to verify this, we need to make sure that the traffic is not just between two hosts, but also the ports, i.e. the traffic is beginning on the Master server’s MySQL port and culminating at the slave server’s MySQL port. The port is discovered in the discovery phase and this information is stored then.