You can try Fast.com (Netflix) or Cloudflare’s one which are explicitly designed to work around this by serving the test data from the same endpoints the serve actual customer data, so ISPs can’t cheat.

This still doesn’t guarantee however that you will achieve this speed to any random host on the internet - their pipe to Cloudflare/Netflix may very well be fat and optimized but it doesn’t guarantee their pipe to a random small hosting provider doesn’t go over a 56k modem somewhere (I jest.. but only a bit).