how to know what client send emit socket.io code example

Example 1: socket io emit to socket id

io.to(socketid).emit('message', 'for your eyes only');

Example 2: socket io emit to specific client

Let me make it simpler with socket.io rooms. request a server 
with a unique identifier to join a server. here we are using 
an email as a unique identifier.

Client Socket.io
socket.on('connect', function () {
  socket.emit('join', {email: [email protected]});
});
When the user joined a server, create a room for that user

Server Socket.io
io.on('connection', function (socket) {
   socket.on('join', function (data) {    
    socket.join(data.email);
  });
});
Now we are all set with joining. let emit something to from the 
server to room, so that user can listen.

Server Socket.io
io.to('[email protected]').emit('message', {msg: 'hello world.'});

Let finalize the topic with listening to message event to the 
client side
socket.on("message", function(data) {
  alert(data.msg);
});

The reference from Socket.io rooms