Автор: http://www.swissdelphicenter.ch

// if the result is 0, no mouse is present 
// wenn result 0 zuruckgibt, ist keine Maus angeschlossen. 

function GetNumberOfMouseButtons: Integer;
 begin
   Result := GetSysTemMetrics(SM_CMOUSEBUTTONS);
 end;

 procedure TForm1.Button1Click(Sender: TObject);
 begin
   ShowMessage('Your mouse has ' + IntToStr(GetNumberOfMouseButtons) + ' buttons.');
 end;