This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Assuming you have a connection via Socket, you can look at the socket class, you will see a method called getRemoteSocketAddress that returns a SocketAddress which will print the IP address and port of the other endpoint. That is probably the most direct way.
"Computer science is no more about computers than astronomy is about telescopes" - Edsger Dijkstra
subject: NetworkInterface.getByInetAddress() in a remote computer