See the "powered by Subversion" line when accessing the server via a browser.
Access the repository via browser and then look for the version string embedded in the HTML source. From earlier answers by elviejo and jaredjacobs. Similarly, from ??, use your browser's developer tools (usually Ctrl + Shift + I) to read the full response. This is also the easiest (non-automated) way to deal with certificates and authorization - your browser does it for you.