A sitemap simply tells you what pages exist, not what's on those pages.
Systems such as lunr.js are closer in spirit to a site-oriented search index, though that's not how they're presently positioned, but instead offer JS-based, client-implemented site search for otherwise static websites.
The basic principle of auditing is to randomly sample results. BlackHat SEO tends to rely on volume in ways that would be very difficult to hide from even modest sampling sizes.
Systems such as lunr.js are closer in spirit to a site-oriented search index, though that's not how they're presently positioned, but instead offer JS-based, client-implemented site search for otherwise static websites.
https://lunrjs.com