Following block diagram shows somewhat simplified algorithm of single proxy server testing.
See details of test target selection are available here
Proxy Test via HTTP
After the test target is selected a HTTP GET request is sent via proxy server.
Proxy Test via SOCKS
If the HTTP GET test returned error then Proxy Switcher will attempt to detect if proxy server responds to SOCKS protocol requests and what protocol version does it require.
HTTP GET request is sent via proxy server to the CORE test servers and depending on the response anonymity level
is determined. For most part it looks for your IP address in the forwarded HTTP request headers.
Test POST support
HTTP POST request is sent via proxy server to determine if proxy server allows them to be made.
Proxy Test via HTTPS
HTTP CONNECT request is sent to proxy server. If the request succeeds and a connection with the target server is established, then a SSL/TLS neogation is attempted. After SSL/TLS connection is established HTTP GET request is sent to the connected web server.