The classic: ping the endpoint address, then “ping” the code. If the IP address comes back in 30ms but the JavaScript responds in 330ms, then the client is probably 300ms further away than they say they are claiming.

Or they’re on a computer that’s doing something intensive, swapping, or in low power mode. Or they have a browser extension that does stuff before scripts run. Or their cache is emptier than usual and they’re spending a long time doing an initial fetch of the latest ad tracking package your site installed. The list goes on. That’s far too noisy a signal to decide block-or-not based on. Good enough to try to sell someone faster gear, maybe, but not more than that.