> How to make sure that my squid is NOT Open Proxy? ..

Squid is not a open proxy means,
IF you are allowing only the specified network people to access the squid for the
net usage.

If you have an acl network src /mask-address
http_access deny !network

It is not a open proxy.
Put the acl in the beginning of the squid.conf file to achieve the "Specific Proxy to the network"