[Serbish]

Put this under OnPlayerConnect

Code:

new string[256];
new playerip[24];
format(string,256,"Your IP is: %d",GetPlayerIp( playerid, playerip, 24 ));
SendClientMessage(playerid,COLOR_YELLOW,string);

and look for your IP if it's different than others or just use SendClientMessagToAll to check everyone's.