Alternatively, open up developer tools, go to the network panel, right-click on the headings and enable the protocol field. It will tell you which protocol was used for each request made by your browser. Obviously the server also needs to support HTTP/3 for a request to use HTTP/3.
Another user here commented that it's been enabled randomly for a subset of users. But not for me. And there's nothing on the web I've found about how to enable it. It was once on the Experimental menu, but no longer.
You can enable http/3 just fine even if you also serve clients that are falling behind on web standards, as long as you also serve http/2 as a fallback.
Safari not supporting isn't a problem unless your business only and specifically targets Safari, and even then it's a relatively painless way to help the few customers who run preview versions get a nicer experience.
