Is there a way to know when a specific search result is a best bet

To my knowledge there is not but can see it being a useful feature!

Maybe there is heuristic approach, whereby the boost on the matched term field is so ridiculously high, you can safely group the bosted results as different from normal results from a numerical calculation. I wanted to see if there was an easy/known approach here. I haven't yet dug into the internals of how best bet works yet, but I presume architecturally it is similar to how other standard boosts work (maybe not though too).

I was looking at this this morning. The module page mentions that it applies a class to results when using Search API Pages, and shows how to get the 'elevated' status form inside the set of results (i.e. for each item).

I needed a class on the view row, and handled it like this (in a .theme file):

use Drupal\views\ViewExecutable;

/**
 * Implements hook_views_pre_render()
 */
function mytheme_views_pre_render(ViewExecutable $view) {
  $view_id = $view->id();
  if ($view_id === 'sitewide_search' && $view->current_display === 'sitewide_search_page') {
    // hook_views_pre_render() appears to runs once per view (?); here we use
    // drupal_static to do our part only once.
    $processed = &drupal_static(__FUNCTION__, []);
    $cache_key = sprintf('%s:%s', $view_id, $view->current_display);

    if (isset($processed[$cache_key])) {
      return;
    }

    if ($view->query instanceof \Drupal\search_api\Plugin\views\query\SearchApiQuery) {
      $results = $view->query->getSearchApiResults();
      $result_items = $results->getResultItems();

      foreach ($view->result as $index => $row) {
        if (isset($result_items[$row->search_api_id])) {
          $row->bestbet_elevated = (bool) $result_items[$row->search_api_id]->getExtraData('elevated');
        }
      }
    }

    $processed[$cache_key] = TRUE;
  }
}

/**
 * Implements template_preprocess_views_view_list().
 */
function mytheme_preprocess_views_view_list(&$variables) {
  $view = $variables['view'];

  if ($variables['view']->id() === 'sitewide_search' && $variables['view']->current_display === 'sitewide_search_page') {
    foreach ($variables['rows'] as $index => &$row) {
      $result_row = $variables['view']->result[$index];

      if ($result_row->bestbet_elevated) {
        $row['attributes']->addClass('search-api-elevated');
      }
    }
  }
}

The mytheme_views_pre_render() implementation tags the row as elevated. After that, pretty much anything is possible. For our purposes, we only needed to render a class on that basis, but it should be possible to do other things like render the elevated item using a different view mode etc.