If I stumble upon a Joomla website ( which I have no access to either the server code, nor the admin backend), can I tell what version of Joomla it's on?
Are you referring to your own website or an external website. If external, then this cannot be done
But on the other hand, I also encounter a Google Chrome extension that allows one to determine the Joomla version of any external Joomla website. So I think this is not impossible.
So can it be done by just accessing the HTML, JS, CSS files of the website and all of the tools that are available to a normal but internet savvy user?