If you do not have physical/remote access to log into the machine, the first thing to try is simply the name or address of the machine. It is likely that the full sql server is installed as the default instance, which is the name of the host.
If this does not work and you do have access to the machine you can open any of the configuration tools, or admin tools>computer management>services>sql services
or you can drill into the registry
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\Instance Names\SQL
Or, you could just ask the administrator. ;-)