There’s a handy command-line tool called is-http which is installed using npm as follows:
npm install -g is-http2-cli
Once installed you can check the HTTP/2 status of a web on the command-line:
$ is-http2 www.atomictag.com ✓ HTTP/2 supported by www.atomictag.com Supported protocols: h2 spdy/3.1 http/1.1
The is-http tool is also useful because it gives you a list of the protocols advertised by the server. As you can see www.cloudflare.com supports HTTP/2, HTTP/1.1 and SPDY/3.1.