[[ArdiIIa]]

Código Delphi [-]
function IsAdmin: Boolean;
const
  SECURITY_NT_AUTHORITY: TSIDIdentifierAuthority =
    (Value: (0, 0, 0, 0, 0, 5));
  SECURITY_BUILTIN_DOMAIN_RID = $00000020;
  DOMAIN_ALIAS_RID_ADMINS = $00000220;
var
  hAccessToken: THandle;
  ptgGroups: PTokenGroups;
  dwInfoBufferSize: DWORD;
  psidAdministrators: PSID;
  x: Integer;
  bSuccess: BOOL;
begin
  Result   := False;
  bSuccess := OpenThreadToken(GetCurrentThread, TOKEN_QUERY, True,
    hAccessToken);
  if not bSuccess then
  begin
    if GetLastError = ERROR_NO_TOKEN then
      bSuccess := OpenProcessToken(GetCurrentProcess, TOKEN_QUERY,
        hAccessToken);
  end;
  if bSuccess then
  begin
    GetMem(ptgGroups, 1024);
    bSuccess := GetTokenInformation(hAccessToken, TokenGroups,
      ptgGroups, 1024, dwInfoBufferSize);
    CloseHandle(hAccessToken);
    if bSuccess then
    begin
      AllocateAndInitializeSid(SECURITY_NT_AUTHORITY, 2,
        SECURITY_BUILTIN_DOMAIN_RID, DOMAIN_ALIAS_RID_ADMINS,
        0, 0, 0, 0, 0, 0, psidAdministrators);
      {$R-}
      for x := 0 to ptgGroups.GroupCount - 1 do
        if EqualSid(psidAdministrators, ptgGroups.Groups[x].Sid) then
        begin
          Result := True;
          Break;
        end;
      {$R+}
      FreeSid(psidAdministrators);
    end;
    FreeMem(ptgGroups);
  end;
end;


procedure TForm1.Button4Click(Sender: TObject);
begin
   if isAdmin then
    ShowMessage('Es Administrador');
end;

Adicionalmente, tenemos este otro procedimiento en el que hemos omitido las variables globales, y seguramente no necesitemos el SP2.

Fuente Torry's