Simple, cute solution that anybody can understand: record yourself setting it up. A video feed from a camera and another from the screen itself. Starting from

Simple, cute solution that anybody can understand: record yourself setting it up. A video feed from a camera and another from the screen itself. Starting from a fresh system, install the dependencies, download the source, verify the hashes, and run the server. Then show the server response from another machine. Sure it doesn't guarantee 100%. The OS image could have been tampered, or you mucked with the network and intercepted requests. But it's easy to record, easy to understand and much more secure than a black box server.