This is very easy, what you have to do is use curl to access any site, and curl will let you know what is the proxy address if you have not put in your authentication details. See below example:
foobar@ubuntu-vm:~$ curl www.google.com -IL HTTP/1.1 302 authenticationrequired Date: Tue, 26 Jan 2016 06:46:15 GMT Location: http://10.155.120.23:9090/mwg-internal/de5fs23hu73ds/plugin?target=Auth&reason=Auth&ClientID=2297288988&ttl=600&url=aHR0cDovL3d3dy5nb29nbGUuY29tLw==&rnd=1453790775 Connection: Keep-Alive Content-Type: text/html Cache-Control: no-cache
HTTP/1.1 401 authenticationrequired Date: Tue, 26 Jan 2016 06:46:15 GMT Connection: Keep-Alive Content-Type: text/html Cache-Control: no-cache WWW-Authenticate: NTLM WWW-Authenticate: Basic realm="MY COMPANY WEB GATEWAY"
You can see from above that my proxy ip address is 10.155.120.23, and my proxy port is 9090.
