[JohnFitzgeraldKennedy]

You would either need some sort of an IP leak vuln in CS, which im not aware of (but I also havent looked into the CS network protocol at all), if the protocol is using a dedicated server. If its peer to peer you can get the IP, if you are the host, for all players in the lobby/game using something like wireshark. Otherwise you'd need to do some sort of social engineering and get someone to visit a host under your control, which would reveal their IP (presuming no VPN/tor/etc.)