The PageSpeed & Core Web Vitals test runs a real Google Lighthouse audit on your site and returns scores for Performance, SEO, Accessibility and Best Practices, plus your Core Web Vitals. Where a domain does not have enough real-user CrUX field data, the tool falls back to lab data and clearly labels it as such, so you are never reading a fallback number as if it were live-traffic data.
Run it on your homepage and your key landing pages — Core Web Vitals affect both search ranking and conversion rate, so a slow page is a growth problem, not just a technical one.