How to find out if the logged in user is a sysadmin member

'sysadmin' is one of the fixed server roles. As stated in SQL Server 2005 Books Online, each member of a fixed server role can add other logins to that same role.

The following query returns the list of available fixed server roles
EXEC sp_helpsrvrole;

Members belonging to the sysadmin server role have the permissions to perform any activity in the server.

In order to find out if the logged in user is a sysadmin:
SELECT IS_SRVROLEMEMBER( 'sysadmin', '' );

Returns 1 if user is a sysadmin, 0 if user is not.

In order to find out the permissions associated with the sysadmin fixed server role:
EXEC sp_srvrolepermission 'sysadmin';