how can i check whethar session is set or not using constructor of controller in laravel framework??

This UserController that contain do_login Function with session which i have already set now problem is how to check whether session is set or not ??

**This is my controller where i want to define constructor that will check whether the session is set or not ..**class UsersController extends Controller
{
  public function do_login(){
    $data['posts'] = Request::all();
    // print_r($data['posts']);
     //exit;
       $data['result'] = Login_Model::login($data);
      //print_r($data['result'][0]->name);



      //exit;
     if(count($data['result'])>0)
     {
           $username = $data['result'][0]->name;
           $userid = $data['result'][0]->id;
            session()->put('username',$username);
            session()->put('userid',$userid);
           //echo "<pre>";print_r($data['result'][0]);exit;
            $user_data = session()->get('username');
        if(isset($user_data) || $user_data != "")
        {
            Session::flash('message', 'Successfully Logged In!');
            return Redirect::to('content');
        }   
        else
        {
            Session::flash('message', 'Logged In Failed!');
            return Redirect::to('register');
        }
    }
    else
    {
        Session::flash('message', 'Logged In Failed!');
            return Redirect::to('register');
    }    
 }

via Chebli Mohamed