php laravel 判断是否存在

Question

 public function postCollect(Request $request)

    {
            $question= new Question;
            $question_list->uesr_id = Auth::id();
            $question_list->question_id = Input::get('question_id')
            $question_list->question_answer = Input::get('question_answer');

            $question_list->save();

              return "success";
        }