Perform a cross-site request back to samesitetest.com to test the SameSite cookie attribute:
These links/button initiate a cross-site request back to samesitetest.com,
which will allow you to see which cookies the browser adds to the request.
Your Browser Developer Tools will allow you to monitor the cookies being sent.
An experiment by Stephen Rees-Carter for "CSRF is dead (or is it?)" and "SameSite Cookies Deep Dive".
Source Code on GitHub, PRs welcome.