The Windows print spooler regularly broadcasts a system wide WM_SPOOLERSTATUS message each time a job is added or deleted from the spooler que. The following example demonstrates trapping for this message.

Example:
type
  TForm  = class(TForm)
    Label : TLabel;
  private
    { Private declarations }
    procedure WM_SpoolerStatus(var Msg : TWMSPOOLERSTATUS);
      message WM_SPOOLERSTATUS;
  public
    { Public declarations }
  end;
var
  Form : TForm ;
implementation
{$R *.DFM}
procedure TForm .WM_SpoolerStatus(var Msg : TWMSPOOLERSTATUS);
begin
  Lable .Caption := IntToStr(msg.JobsLeft) +
                    ' Jobs currenly in spooler';
  msg.Result := 0;
end;