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Cuando cambias de impresora con el PrinterSetupDialog, este cambia la impresora por defecto, y puedes averiguar cuál es la impresora actual utilizando el objeto/función Printer de la unit printers:

Código Delphi [-]
uses
  Printers;

procedure TForm1.Button1Click(Sender: TObject);
var
  ADevice, ADriver, APort: PChar;
  ADeviceMode: DWORD;
begin
  if PrinterSetupDialog1.Execute then
  begin
    ADevice := StrAlloc(MAX_PATH);
    ADriver := StrAlloc(MAX_PATH);
    APort := StrAlloc(MAX_PATH);
    try
      Printer.GetPrinter(ADevice, ADriver, APort, ADeviceMode);
      with Memo1.Lines do
      begin
        Add('Device: ' + string(ADevice));
        Add('Driver: ' + string(ADriver));
        Add('Port: ' + string(APort));
        Add('DeviceMode: ' + IntToStr(ADeviceMode));
      end;
    finally
      StrDispose(ADevice);
      StrDispose(ADriver);
      StrDispose(APort);
    end;
  end;
end;

Saludos!