Sites don't need to know your real identity to suspect you're using a proxy — a handful of technical signals give it away consistently.
Known IP ranges
Datacenter and free proxy IP ranges are well documented and shared across security vendors, so many sites simply check incoming IPs against these known blocklists before anything else happens.
Header inconsistencies
Even anonymous proxies sometimes leave subtle header signatures, and any proxy that isn't truly elite may pass along headers like X-Forwarded-For that immediately flag the traffic as proxied.
Behavioral signals
Beyond IP and headers, sites look at request timing, whether JavaScript executes normally, mouse movement on interactive pages, and how closely traffic patterns resemble a real browsing session versus an automated script.
The response when detected
Detected proxy traffic commonly gets a CAPTCHA challenge rather than an outright block, since sites want to filter bots without turning away real users on legitimate VPNs or corporate proxies — passing the CAPTCHA is often enough to proceed.