Test for breakage

You can launch Chrome using the --test-third-party-cookie-phaseout command-line flag or from Chrome 118, enable chrome://flags/#test-third-party-cookie-phaseout. This will set Chrome to block third-party cookies and ensure that new functionality and mitigations are active in order to best simulate the state after the phase out.

You can also try browsing with third-party cookies blocked via chrome://settings/cookies, but be aware that the flag ensures the new and updated functionality is also enabled. Blocking third-party cookies is a good approach to detect issues, but not necessarily validate you have fixed them.

If you maintain an active test suite for your sites, then you should do two side-by-side runs: one with Chrome on the usual settings and one with the same version of Chrome launched with the --test-third-party-cookie-phaseout flag. Any test failures in the second run and not in the first are good candidates to investigate for third-party cookie dependencies. Make sure you report the issues you find.