In some cases it may not be clear which socket is the master socket, especially if the master socket is one of the old non-NTE5 faceplates.
The best way of finding the master socket is looking at each socket in the home and if it's an NTE5 then it will be the master socket. An NTE5 socket will look like the following:
If you are still having trouble locating the master socket then if you look on the outside of your property and you can see the phone line coming into the property the master socket will be near to where it enters.
If your all of your sockets are non-NTE5 and the location isn't obvious from where the line comes into your property then it would be a case of removing the faceplate of each socket and the master socket will have a capacitor, similar to the the following image: