code: protected function authenticate()
      {
          if (empty($this->guards) && Auth::check()) {
              return;
          }
  
          foreach ($this->guards as $guard) {
              if (Auth::guard($guard)->check()) {
                  Auth::shouldUse($guard);
                  return;
              }
          }
  
          throw new AuthenticationException('Unauthenticated.', $this->guards);
      }
docs: Determine if the user is logged in.
  
  @return void
  @throws AuthenticationException