Is there a way in drupal 8 to tell if the current page is a node?

    \Drupal::request()->attributes->get('node');

Taken from here: https://api.drupal.org/api/drupal/core!lib!Drupal.php/function/Drupal%3A%3Arequest/8

Now the docs say to never use this function, but I have used this to determine what kind of page I am on.

There is another function, but I am not sure it applies in your case:

https://api.drupal.org/api/drupal/core%21modules%21node%21node.module/function/node_is_page/8

At the very least, if you do not have an entity, you could mimic that in your code:

      $route_match = \Drupal::routeMatch();
      if ($route_match->getRouteName() == 'entity.node.canonical') {
        return true;
      }

Reference