Most often the answer is "Nope" - indicating that Varnish isn't operating - even thought the cache is running. I checked I can work around that - maybe they're doing some simple check that can be passed with a minor modification to the VCL file.
Unfortunately, I've found out that the data presented on this website is completely useless - the headers they show to visitors are different from actual headers sent by the server!
You can see that yourself below:
|The headers isvarnishworking.com shows|
|Actual headers, as seen by wget|
I don't understand what's the point of building a service showing fake HTTP headers.
When testing, just use the Network tab in developer tools in your browser, or wget/wbox if you're a command line user.