You mean this? http://whatismyipaddress.com/proxy-check
Could have been more clear... to bypass it, I guess use a better proxy?

I'm sure it works by checking public sources of proxy addresses, checking for common proxy software being present on port scans or other on analytical sources.


But this is a completely closed source solution, people have requested access to use it for other applications or from the API and have been declined, so I'm sure that there is some black magic in use that they don't want to get out.