I have created a new form mode (in the UI at
admin/structure/display-modes) for a custom entity (form mode is entitled
apply) and added
apply to the form handler classes in the entity class.
The form is built and returned in a Controller, and specifies the form mode.
$form = $this->entityFormBuilder()->getForm($entity, 'apply');
Now I would like to be able to determine from a
hook_form_alter whether this form is being displayed using the
apply form mode, but I cannot work out how that is available from