Im Doing a Boxing game, and when a player loose, I need his ID to share with the server and the other player can celebrate.
I can’t test this at the moment, but if you know the player object of the losing player, you can use playerObject.GetComponent(NetworkIdentity).connectionToClient.connectionID to check that players connection ID. ( http://docs.unity3d.com/ScriptReference/Networking.NetworkIdentity-connectionToClient.html )
Alternatively if you have any message communication with that player, you can always get the connectionID from NetworkMessage.conn.connectionID.