How do the websites know I am using Proxy and sometimes VPN?

Hi guys,

Recently I went on to some sites by using web proxy extension on chrome but the website detected that I am using proxy and I wan unable to sign-up. Whats the reason behind that and how that website know ?

There are many APIs that can detect VPN or proxy.
Basically Residential IPs are good


One website the provides a really good API is .
Before going to the website check ur IP if its residential u are good to go.

For a fact i know that most IPs are residential. They provide a free browser extension.