Sockets - check connection by client.

Hey guys. I created a multiple client/server application using sockets in c#. Everything works just fine. Now can you please help me create a code that will allow a client to check if another client is connected to the server(By IP)? I have an idea but I don't know how to implement it. I want something like that: Client #1 has connected. Client #2 has connected. Client #1 sends to server: "check 45.56.128.20" if client #2 is connected, do nothing. Else: Server sends to Client #1: "The client is not connected". My server size code: http://pastebin.com/vmwJFxFt My client side code: http://pastebin.com/YD28bNCg (I did not write the server&client codes) Thank you!