Core Web Vitals consist of three metrics: Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS). You measure them through field data from real users or lab tests that simulate conditions. The fastest way is Google PageSpeed Insights. Enter your URL and you'll get both field data (actual Chrome users over the past 28 days) and lab data (simulated on a throttled connection). Field data is what Google uses for ranking, so prioritize those numbers. You want LCP under 2.5 seconds, INP under 200ms, and CLS under 0.1 to pass. Between 2.5-4.0s LCP, 200-500ms INP, or 0.1-0.25 CLS means you need improvement. Search Console's Core Web Vitals report groups your URLs into Good, Needs Improvement, and Poor buckets. This shows which page types have problems at scale. If your product pages fail but blog posts pass, you know where to focus. The report uses Chrome User Experience Report data, so you need actual traffic to populate it. For ongoing monitoring, Chrome DevTools Performance panel lets you record page loads and see exactly when LCP occurs, which elements shift (causing CLS), and where interaction delays happen. The Lighthouse tab in DevTools runs the same engine as PageSpeed Insights but locally. We run Lighthouse CI in deployment pipelines to catch regressions before they hit production. Third-party tools like WebPageTest give more granular control over test locations and connection speeds. You can test from Ottawa, Toronto, or international nodes to see how CDN performance varies. For portfolio-scale monitoring across 500+ domains, we use CrUX API to pull monthly field data programmatically and flag domains falling below thresholds. The critical distinction: lab data is consistent and debuggable but artificial. Field data reflects real users on real devices and connections, but updates slowly and needs traffic volume. At Ottawa SEO, we fix issues using lab tools but validate success with field data over 3-4 weeks. Don't obsess over a single test run; variation of 10-15% between tests is normal.