PageSpeed Insights
PageSpeed Insights is Google's free tool that analyses a given URL and shows its Core Web Vitals values, both from real user data where available and from a simulated lab test, along with concrete suggestions for improvement.
In practice
The lab score can differ from the real field score, because the field data reflects the actual devices and connections of real visitors. What counts for Google rankings in the end is the field data, not the lab result. In practice it is worth checking your most important pages in PageSpeed Insights regularly, rather than relying on a single measurement.