Drupal 8 - detect if we're in an ajax request, and if building the form for the first time, or re-building within the input processing.

example_ajax.php

<?php

use Drupal\Core\Form\FormStateInterface;

function example_ajax_form_alter() {
  $ajax_form_request = \Drupal::request()->query->has(FormBuilderInterface::AJAX_FORM_REQUEST);
  if ($ajax_form_request) {
    if (!$form_state->isProcessingInput()) {
      \Drupal::logger('example')->notice('first pass');
    }
    else {
      \Drupal::logger('example')->notice('second pass');
    }
  }
}

That worked great @lonalore, thanks!