Drupal 8 FORM ALTER – How do I know if the form is being assembled for viewing.

I’m trying to display a message (drupal_set_message()) when the form I’m altering (node edit form) is being viewed.
Unfortunately the message is also displayed after the form has been submitted.

I’d add a condition but I can’t quite figure out “where I am” in the form workflow.

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/4/feed