You don't have to perform any task (destructive or non-destructive) at all.

You can use the built-in function IS_SRVROLEMEMBER to find if a certain LOGIN is a memeber of the sysadmin server role:

SELECT IS_SRVROLEMEMBER('sysadmin','<LoginName>');

Note that for roles other than sysadmin the result will be positive (=1) for implicit membership as well.

You can use the built-in stored procedure sp_helpsrvrolemember to get a list of all the explicit members of a certain role:

EXEC sys.sp_helpsrvrolemember @srvrolename = 'sysadmin';

Or a complete list of all explicit members of all server roles:

EXEC sys.sp_helpsrvrolemember;

For SQL Server 2000 there is also the option to sort or filter the result set using:

CREATE TABLE #ServerRoles 
( 
  ServerRole VARCHAR(20), 
  MemberName sysname, 
  sid VARBINARY(85) 
); 

INSERT INTO #ServerRoles 
(ServerRole, MemberName, sid) 
EXEC sys.sp_helpsrvrolemember;

SELECT SR.MemberName, SR.ServerRole 
FROM #ServerRoles SR 
WHERE 1 = 1                            -- Change this to any filter you want
ORDER BY SR.MemberName, SR.ServerRole; -- Change this to any sorting you want

DROP TABLE #ServerRoles;
GO

For SQL Server 2005+ there is already a system view to be queried directly:
sys.server_role_members which is a many-to-many relationship table between roles and principals.

You don't have to perform any task (destructive or non-destructive) at all.

You can use the built-in function IS_SRVROLEMEMBER to find if a certain LOGIN is a memeber of the sysadmin server role:

SELECT IS_SRVROLEMEMBER('sysadmin','<LoginName>');

Note that for roles other than sysadmin the result will be positive (=1) for implicit membership as well.

You can use the built-in stored procedure sp_helpsrvrolemember to get a list of all the explicit members of a certain role:

EXEC sys.sp_helpsrvrolemember @srvrolename = 'sysadmin';

Or a complete list of all explicit members of all server roles:

EXEC sys.sp_helpsrvrolemember;

For SQL Server 2000 there is also the option to sort or filter the result set using:

CREATE TABLE #ServerRoles 
( 
  ServerRole VARCHAR(20), 
  MemberName sysname, 
  sid VARBINARY(85) 
); 

INSERT INTO #ServerRoles 
(ServerRole, MemberName, sid) 
EXEC sys.sp_helpsrvrolemember;

SELECT SR.MemberName, SR.ServerRole 
FROM #ServerRoles SR 
WHERE 1 = 1                            -- Change this to any filter you want
ORDER BY SR.MemberName, SR.ServerRole; -- Change this to any sorting you want

DROP TABLE #ServerRoles;
GO

For SQL Server 2005+ there is already a system view to be queried directly:
sys.server_role_members which is a many-to-many relationship table between roles and logins.