Grav plugin/function to determine whether there is an explicit summary in the page content. This allows you to safely slice the summary off using "{{ content|slice(page.summary|length)|raw }}" if "page.header.has_explicit_summary" is true.

summary-explicitness.php

<?php
namespace Grav\Plugin;

use Grav\Common\Plugin;
use \RocketTheme\Toolbox\Event\Event;

class SummaryExplicitnessPlugin extends Plugin
{
  public static function getSubscribedEvents() 
  {
    return [
      'onPageContentRaw' => ['setSummaryExplicitness', 0],
    ];
  } 

  /**
   * Check whether the page has an explicit summary
   * 
   * Based on: https://discourse.getgrav.org/t/page-with-or-without-summary/2611/12
   */
  public function setSummaryExplicitness(Event $event)
  {
    // Get the current raw content
    $page = $event['page'];
    $raw = $page->getRawContent();

    $summaryseparator = $this->config->get("site.summary.delimiter");
    $summaryseparatorpos = mb_strpos($raw, $summaryseparator);

    if ($summaryseparatorpos) {
      $page->header()->has_explicit_summary = true;
    }
    else {
      $page->header()->has_explicit_summary = false;
    }
  }
}

I've reformatted this as a plugin to share here, but I actually include the function within my theme, since the theme relies on it.

To utilise this in a page template, use something like:

{% if page.header.has_explicit_summary %}
  {{ content|slice(page.summary|length)|raw }}
{% else %}
  {{ content|raw }}
{% endif %}