How can I know that my data is not being shared with others behind the scenes? This is a legitimate concern for many. Here I provide ideas and tools for how you can verify that your data is not being shared without your permission.
Lock down and isolate your computer from any network connections.
One approach is to ensure your computer is not connected to the internet. Do not connect a network cable to your computer and turn Wi-Fi and Bluetooth connections off in your computer settings. This is generally referred to as an "air gapped" computer.
This is a great approach that some people I know use. They keep a computer dedicated to managing their collection isolated from all networks and other computers. Some of them use a notebook computer that they keep locked in a safe to minimize the risk that someone will access the computer directly.
Configure a firewall to block all external connections.
Baring complete isolation from any networks, another approach might be to block all outbound connections from your computer to the outside world using a firewall. There are many ways to do this. Here is an example using Windows Firewall. Here is an example for Mac Computers. There are also many third-party tools for this.
More advanced techniques
There are actually tools available to monitor incoming and outgoing connections. These are better left to the networking experts.