random technical notes

The CheckTokenMembership function determines whether a specified security identifier (SID) is enabled in an access token.

/*
Routine Description: This routine returns TRUE if the caller's
process is a member of the Administrators local group. Caller is NOT
expected to be impersonating anyone and is expected to be able to
open its own process and process token.
Arguments: None.
Return Value:
TRUE - Caller has Administrators local group.
FALSE - Caller does not have Administrators local group.
*/
BOOL IsUserAdmin(void)
{
 BOOL bResult;
 SID_IDENTIFIER_AUTHORITY NtAuthority = SECURITY_NT_AUTHORITY;
 PSID AdministratorsGroup;
 bResult = AllocateAndInitializeSid(&NtAuthority, 2, SECURITY_BUILTIN_DOMAIN_RID,
                                    DOMAIN_ALIAS_RID_ADMINS, 0, 0, 0, 0, 0, 0,
                                    &AdministratorsGroup);
 if(bResult)
 {
  if(!CheckTokenMembership(NULL, AdministratorsGroup, &bResult))
  {
   bResult = FALSE;
  }
  FreeSid(AdministratorsGroup);
 }
 return(bResult);
}

MSDN