How to tell if the current revision is the original?
NodeStorage::revisionIds() will let you retrieve all revisions of a node which will let you compare against the current node's version.

 // Retrieve all vids.
 $nid = 123;
 $node = Drupal\node\Entity\Node::load($nid);
 $vids = \Drupal::entityTypeManager()-&gt;getStorage('node')-&gt;revisionIds($node);

 // Does the original node's vid match the current node's vid?
 return (reset($vids) == $node-&gt;vid-&gt;getString());